[edk2-devel] A problem with live migration of UEFI virtual machines
On 02/24/20 16:28, Daniel P. Berrangé wrote:
On Tue, Feb 11, 2020 at 05:39:59PM +0000, Alex Bennée wrote:Following up here *too*, just for completeness.I don't believe we are that strict for firmware in general. The
The query in this thread has been posted three times now (and I have
zero idea why). Each time it generated a different set of responses. For
completes, I'm now going to link the other two threads here (because the
present thread seems to have gotten the most feedback).
To the OP:
- please do *NOT* repost the same question once you get an answer. It
only fragments the discussion and creates confusion. It also doesn't
hurt if you *confirm* that you understood the answer.
- Yet further, if your email address has @gmail.com for domain, but your
msgids contain "tencent", that raises some eyebrows (mine for sure).
You say "we" in the query, but never identify the organization behind
the plural pronoun.
(I've been fuming about the triple-posting of the question for a while
now, but it's only now that, upon seeing how much work Dan has put into
his answer, I've decided that dishing out a bit of netiquette would be
* First posting:
- msgid: <tencent_F1295F826E46EDFF3D77812B@...>
- edk2-devel: https://edk2.groups.io/g/devel/message/54146
- qemu-devel: https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg02419.html
* my response:
- msgid: <firstname.lastname@example.org>
- edk2-devel: https://edk2.groups.io/g/devel/message/54161
- qemu-devel: none, because (as an exception) I used the stupid
groups.io web interface to respond, and so my response
never reached qemu-devel
* Second posting (~4 hours after the first)
- msgid: <tencent_3CD8845EC159F0161725898B@...>
- edk2-devel: https://edk2.groups.io/g/devel/message/54147
- qemu-devel: https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg02415.html
* Dave's response:
- msgid: <20200220154742.GC2882@work-vm>
- edk2-devel: https://edk2.groups.io/g/devel/message/54681
- qemu-devel: https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg05632.html
* Third posting (next day, present thread) -- cross posted to yet
another list (!), because apparently Dave's feedback and mine had not
- msgid: <tencent_BC7FD00363690990994E90F8@...>
- edk2-devel: https://edk2.groups.io/g/devel/message/54220
- edk2-discuss: https://edk2.groups.io/g/discuss/message/135
- qemu-devel: https://lists.gnu.org/archive/html/qemu-devel/2020-02/msg02735.html
Back on topic: see my response again. The answer is, you can't solve the
problem (specifically with OVMF), and QEMU in fact does you service by
preventing the migration.
On 02/25/20 19:56, Andrew Fish wrote:
The legacy BIOS used fixed magic address ranges, but UEFI uses dynamically allocated memory so addresses are not fixed. While the UEFI firmware does try to keep S3 and S4 layouts consistent between boots, I'm not aware of any mechanism to keep the memory map address the same between versions of the firmware?It's not about RAM, but platform MMIO.
The core of the issue here is that the -D FD_SIZE_4MB and -D FD_SIZE_2MB
build options (or more directly, the different FD_SIZE_IN_KB macro
settings) set a bunch of flash-related build-time constant macros, and
PCDs, differently, in the following files:
As a result, the OVMF_CODE.fd firmware binary will have different
hard-coded references to the variable store pflash addresses.
(Guest-physical MMIO addresses that point into the pflash range.)
If someone tries to combine an OVMF_CODE.fd firmware binary from e.g.
the 4MB build, with a variable store file that was originally
instantiated from an OVMF_VARS.fd varstore template from the 2MB build,
then the firmware binary's physical address references and various size
references will not match the contents / layout of the varstore pflash
chip, which maps an incompatibly structured varstore file.
For example, "OvmfPkg/VarStore.fdf.inc" describes two incompatible
EFI_FIRMWARE_VOLUME_HEADER structures (which "build" generates for the
OVMF_VARS.fd template) between the 4MB (total size) build, and the
1MB/2MB (total size) build.
The commit message below summarizes the internal layout differences,
from 1MB/2MB -> 4MB:
Excerpt (relevant for OVMF_VARS.fd):
Description Compression type Size [KB]
------------------------- ----------------- ----------------------
Non-volatile data storage open-coded binary 128 -> 528 ( +400)
Variable store 56 -> 256 ( +200)
Event log 4 -> 4 ( +0)
Working block 4 -> 4 ( +0)
Spare area 64 -> 264 ( +200)
On Feb 25, 2020, at 9:53 AM, Laszlo Ersek <lersek@...> wrote:
On 02/25/20 22:35, Andrew Fish wrote:
Laszlo,With live migration, the running guest doesn't notice anything. This is
a general requirement for live migration (regardless of UEFI or flash).
You are very correct to ask about "skipping" the NVRAM region. With the
approach that OvmfPkg originally supported, live migration would simply
be unfeasible. The "build" utility would produce a single (unified)
OVMF.fd file, which would contain both NVRAM and executable regions, and
the guest's variable updates would modify the one file that would exist.
This is inappropriate even without considering live migration, because
OVMF binary upgrades (package updates) on the virtualization host would
force guests to lose their private variable stores (NVRAMs).
Therefore, the "build" utility produces "split" files too, in addition
to the unified OVMF.fd file. Namely, OVMF_CODE.fd and OVMF_VARS.fd.
OVMF.fd is simply the concatenation of the latter two.
$ cat OVMF_VARS.fd OVMF_CODE.fd | cmp - OVMF.fd
When you define a new domain (VM) on a virtualization host, the domain
definition saves a reference (pathname) to the OVMF_CODE.fd file.
However, the OVMF_VARS.fd file (the variable store *template*) is not
directly referenced; instead, it is *copied* into a separate (private)
file for the domain.
Furthermore, once booted, guest has two flash chips, one that maps the
firmware executable OVMF_CODE.fd read-only, and another pflash chip that
maps its private varstore file read-write.
This makes it possible to upgrade OVMF_CODE.fd and OVMF_VARS.fd (via
package upgrades on the virt host) without messing with varstores that
were earlier instantiated from OVMF_VARS.fd. What's important here is
that the various constants in the new (upgraded) OVMF_CODE.fd file
remain compatible with the *old* OVMF_VARS.fd structure, across package
If that's not possible for introducing e.g. a new feature, then the
package upgrade must not overwrite the OVMF_CODE.fd file in place, but
must provide an additional firmware binary. This firmware binary can
then only be used by freshly defined domains (old domains cannot be
switched over). Old domains can be switched over manually -- and only if
the sysadmin decides it is OK to lose the current variable store
contents. Then the old varstore file for the domain is deleted
(manually), the domain definition is updated, and then a new (logically
empty, pristine) varstore can be created from the *new* OVMF_2_VARS.fd
that matches the *new* OVMF_2_CODE.fd.
During live migration, the "RAM-like" contents of both pflash chips are
migrated (the guest-side view of both chips remains the same, including
the case when the writeable chip happens to be in "programming mode",
i.e., during a UEFI variable write through the Fault Tolerant Write and
Firmware Volume Block(2) protocols).
Once live migration completes, QEMU dumps the full contents of the
writeable chip to the backing file (on the destination host). Going
forward, flash writes from within the guest are reflected to said
host-side file on-line, just like it happened on the source host before
live migration. If the file backing the r/w pflash chip is on NFS
(shared by both src and dst hosts), then this one-time dumping when the
migration completes is superfluous, but it's also harmless.
The interesting question is, what happens when you power down the VM on
the destination host (= post migration), and launch it again there, from
zero. In that case, the firmware executable file comes from the
*destination host* (it was never persistently migrated from the source
host, i.e. never written out on the dst). It simply comes from the OVMF
package that had been installed on the destination host, by the
sysadmin. However, the varstore pflash does reflect the permanent result
of the previous migration. So this is where things can fall apart, if
both firmware binaries (on the src host and on the dst host) don't agree
about the internal structure of the varstore pflash.
On 02/28/20 04:20, Zhoujian (jay) wrote:
Hi Laszlo,Yes, exactly.-----Original Message-----Hi Laszlo,
I'm unaware of any VMs running in clouds that use "-bios" with OVMF. It
certainly seems a terrible idea, regardless of live migration.
You're mixing up small details. OVMF_CODE.fd is already heavily padded,
internally. We've grown the *internal* DXEFV firmware volume repeatedly
over *years*, without *any* disruption to users. Please see:
- da78c88f4535 ("OvmfPkg: raise DXEFV size to 8 MB", 2014-03-05)
- 08df58ec3043 ("OvmfPkg: raise DXEFV size to 9 MB", 2015-10-07)
- 2f7b34b20842 ("OvmfPkg: raise DXEFV size to 10 MB", 2016-05-31)
- d272449d9e1e ("OvmfPkg: raise DXEFV size to 11 MB", 2018-05-29)
To this day, i.e., with edk2 master @ edfe16a6d9f8, you can build OVMF
in the default feature configuration [*] for -D FD_SIZE_2MB.
DEFINE SECURE_BOOT_ENABLE = FALSE
DEFINE SMM_REQUIRE = FALSE
DEFINE SOURCE_DEBUG_ENABLE = FALSE
DEFINE TPM2_ENABLE = FALSE
DEFINE TPM2_CONFIG_ENABLE = FALSE
DEFINE NETWORK_TLS_ENABLE = FALSE
DEFINE NETWORK_IP6_ENABLE = FALSE
DEFINE NETWORK_HTTP_BOOT_ENABLE = FALSE
$ build \
-a IA32 -a X64 \
-b DEBUG \
-p OvmfPkg/OvmfPkgIa32X64.dsc \
-t GCC48 \
Note that this build will contain DEBUG messages (at least DEBUG_INFO
level ones) and ASSERT()s too.
The final usage report at the end of the command is:
SECFV [14%Full] 212992 total, 31648 used, 181344 free
PEIFV [31%Full] 917504 total, 284584 used, 632920 free
DXEFV [44%Full] 11534336 total, 5113688 used, 6420648 free
FVMAIN_COMPACT [73%Full] 1753088 total, 1284216 used, 468872 free
What does that mean? It means that largest firmware volume, DXEFV, uses
just 44% of the 11MB allotted size.
And FVMAIN_COMPACT, which embeds (among other things) DXEFV in
LZMA-compressed format, only uses 73% of its allotted size, which is
All this means that in the default feature config, there's still a bunch
of room free in the 2MB build, even with DEBUGs and ASSERT()s enabled,
and with an old compiler that does not do link-time optimization.
I think you must have misunderstood the purpose of the 4MB build. The
4MB build was solely introduced for enlarging the *varstore*. That was
motivated by passing an SVVP check. This is described in detail in the
relevant commit, which I may have linked earlier.
(Please consult the diagram in the commit message carefully. It shows
you how the various firmware volumes / flash devices are nested; it will
help you understand where the 1712 KB FVMAIN_COMPACT firmware volume is
placed in the final image, and how FVMAIN_COMPACT embeds / compresses
And *given that* we had to introduce an incompatible change (for
enlarging the varstore, for SVVP's sake), it made sense to *also*
enlarge the other parts of the flash content. But the motivation was
strictly the varstore change, and that was inevitably an incompatible
change. In fact, you can see in the commit message that the *outer*
container FVMAIN_COMPACT was enlarged from 1712 to 3360 kilobytes, the
embedded PEIFV and DXEFV firmware volumes didn't put that extra space to
use. The SECFV firmware volume runs directly from flash, so it's not
compressed, but even that firmware volume got no "space injection". So
basically all the size increase that *could* have been exploited for
executable code size was spent on padding.
As far as I can tell, we have never broken compatibility due to
executable code size increases.
Sorry if I over-explained this; I simply don't know how to express this
Things are a little different here,No, this doesn't make any sense.
On both the source host and the destination host, the same pathname (for
example, "/usr/share/OVMF/OVMF_CODE.fd") must point to same-size
(compatible) firmware binaries. Both must be built with the same -D
FD_SIZE_2MB flag, or with the same -D FD_SIZE_4MB flag. Then you can
You can offer a 4MB build too on the destination host, but it must be
under a different pathname. So that after the domain has been migrated
in from the source host, and then re-launched against the firmware
binary that's on the destination host, there is an incompatibility
between the domain's *original* varstore, and the domain's *new*
Sorry, my brain just cannot cope with the idea of even *running* OVMF in
production with "-bios" -- let alone migrate it.
But anyway... if you are dead set on this, you can try the following:
- On the destination host, rename the 4MB build to a different filename.
- On the destination host, update all your domain definitions to refer
to the renamed filename with "-bios"
- on the destination host, rebuild your current (more modern) firmware
package, using the -D FD_SIZE_2MB flag. If you have not enabled a bunch
of features meanwhile, it will actually succeed.
- on the destination host, put this fresh build (with unified size 2MB)
in the original place (using the original pathname)
- now you can migrate domains from your source host. The pathname they
refer to with "-bios" will exist, and it will be a 2MB build. And the
contents of that build will be more modern (presumably) than what you
are migrating away from.
Please understand this: when you *allowed* OVMF to build with 4MB size,
and installed it under the exact same pathname (on the destination host)
where you previously used to keep a 2MB binary, *that* is when you broke
What's quite unfathomable to me is that the 2MB->4MB change in upstream
was *solely* motivated by varstore enlargement (for passing SVVP with
*flash*-based variables), but you're still using the ancient and
non-conformant \NvVars emulation that comes with "-bios".
Please, flash based variables with OVMF and QEMU have been supported
since QEMU v1.6.
I've attempted to remove -bios support from OVMF multiple times, I've
always been prevented from doing that, and the damage is obvious only now.
On 02/28/20 05:04, Andrew Fish wrote:
Maybe I was overcomplicating this. Given your explanation I think the part I'm missing is OVMF is implying FLASH layout, in this split model, based on the size of the OVMF_CODE.fd and OVMF_VARS.fd. Given that if OVMF_CODE.fd gets bigger the variable address changes from a QEMU point of view. So basically it is the QEMU API that is making assumptions about the relative layout of the FD in the split model that makes a migration to larger ROM not work.No, QEMU does not make any assumptions here. QEMU simply grabs both
pflash chips (the order is not random, it can be specified on the
command line -- in fact the QEMU user is expected to specify in the
right order), and then QEMU maps them in decreasing address order from
4GB in guest-phys address space.
If we enlarge OVMF_CODE.fd, then the base address of the varstore
(PcdOvmfFlashNvStorageVariableBase) will sink. That's not a problem per
se, because QEMU doesn't know about PcdOvmfFlashNvStorageVariableBase at
all. QEMU will simply map the varstore, automatically, where the
enlarged OVMF_CODE.fd will look for it.
Basically the -pflash API does not support changing the size of the ROM without moving NVRAM given the way it is currently defined.Let me put it like this: the NVRAM gets moved by virtue of how OVMF is
built, and by how QEMU maps the pflash chips into guest-phys address
space. They are in sync, automatically.
The problem is when the NVRAM is internally restructured, or resized --
the new OVMF_CODE.fd binary will reflect this with changed PCDs, and
look for "stuff" at those addresses. But if you still try to use an old
(differently sized, or differently structured) varstore file, while QEMU
will happily map it, parts of the NVRAM will just not end up in places
where OVMF_CODE.fd expects them.
There's already room to grow, *inside* OVMF_CODE.fd. As I've shown
elsewhere in this thread, even the 2MB build has approx. 457 KB free in
the DXEFV volume, even without link-time optimization and without
DEBUG/ASSERT stripping, if you don't enable additional features.
2) Add some feature to QUEM that allows the variable store address to not be based on OVMF_CODE.fd size.Yes, this has been proposed over time.
It wouldn't help with the case when you change the internal structure of
the NVRAM, and try to run an incompatible OVMF_CODE.fd against that.
I did see this  and combined with your email I either understand, or I'm still confused? :)I think the most interesting function for you could be
pc_system_flash_map(), in "hw/i386/pc_sysfw.c", in the QEMU source.
On 02/28/20 12:47, Laszlo Ersek wrote:
On 02/28/20 05:04, Andrew Fish wrote:
Typo; I meant FVMAIN_COMPACT, not DXEFV.Given the above it seems like the 2 options are:There's already room to grow, *inside* OVMF_CODE.fd. As I've shown