Re: [OVMF] resource assignment fails for passthrough PCI GPU

Laszlo Ersek

On 11/22/19 17:58, Dr. David Alan Gilbert wrote:
* Laszlo Ersek ( wrote:
(+Dave, +Eduardo)

On 11/22/19 00:00, dann frazier wrote:
On Tue, Nov 19, 2019 at 06:06:15AM +0100, Laszlo Ersek wrote:
On 11/19/19 01:54, dann frazier wrote:
On Fri, Nov 15, 2019 at 11:51:18PM +0100, Laszlo Ersek wrote:
On 11/15/19 19:56, dann frazier wrote:
I'm trying to passthrough an Nvidia GPU to a q35 KVM guest, but UEFI
is failing to allocate resources for it. I have no issues if I boot w/
a legacy BIOS, and it works fine if I tell the linux guest to do the
allocation itself - but I'm looking for a way to make this work w/
OVMF by default.

I posted a debug log here:

Linux guest lspci output is also available for both seabios/OVMF boots here:
By default, OVMF exposes such a 64-bit MMIO aperture for PCI MMIO BAR
allocation that is 32GB in size. The generic PciBusDxe driver collects,
orders, and assigns / allocates the MMIO BARs, but it can work only out
of the aperture that platform code advertizes.

Your GPU's region 1 is itself 32GB in size. Given that there are further
PCI devices in the system with further 64-bit MMIO BARs, the default
aperture cannot accommodate everything. In such an event, PciBusDxe
avoids assigning the largest BARs (to my knowledge), in order to
conserve the most aperture possible, for other devices -- hence break
the fewest possible PCI devices.

You can control the aperture size from the QEMU command line. You can
also do it from the libvirt domain XML, technically speaking. The knob
is experimental, so no stability or compatibility guarantees are made.
(That's also the reason why it's a bit of a hack in the libvirt domain XML.)

The QEMU cmdline options is described in the following edk2 commit message:
Hi Laszlo,

Thanks for taking the time to describe this in detail! The -fw_cfg
option did avoid the problem for me.
Good to hear, thanks.

I also noticed that the above
commit message mentions the existence of a 24GB card as a reasoning
behind choosing the 32GB default aperture. From what you say below, I
understand that bumping this above 64GB could break hosts w/ <= 37
physical address bits.

What would be the downside of bumping the
default aperture to, say, 48GB?
The placement of the aperture is not trivial (please see the code
comments in the linked commit). The base address of the aperture is
chosen so that the largest BAR that can fit in the aperture may be
naturally aligned. (BARs are whole powers of two.)

The largest BAR that can fit in a 48 GB aperture is 32 GB. Therefore
such an aperture would be aligned at 32 GB -- the lowest base address
(dependent on guest RAM size) would be 32 GB. Meaning that the aperture
would end at 32 + 48 = 80 GB. That still breaches the 36-bit phys
address width.

32 GB is the largest aperture size that can work with 36-bit phys
address width; that's the aperture that ends at 64 GB exactly.
Thanks, yeah - now that I read the code comments that is clear (as
clear as it can be w/ my low level of base knowledge). In the commit you
mention Gerd (CC'd) had suggested a heuristic-based approach for
sizing the aperture. When you say "PCPU address width" - is that a
function of the available physical bits?
"PCPU address width" is not a "function" of the available physical bits
-- it *is* the available physical bits. "PCPU" simply stands for
"physical CPU".

IOW, would that approach
allow OVMF to automatically grow the aperture to the max ^2 supported
by the host CPU?

The current logic in OVMF works from the guest-physical address space
size -- as deduced from multiple factors, such as the 64-bit MMIO
aperture size, and others -- towards the guest-CPU (aka VCPU) address
width. The VCPU address width is important for a bunch of other purposes
in the firmware, so OVMF has to calculate it no matter what.

Again, the current logic is to calculate the highest guest-physical
address, and then deduce the VCPU address width from that (and then
expose it to the rest of the firmware).

Your suggestion would require passing the PCPU (physical CPU) address
width from QEMU/KVM into the guest, and reversing the direction of the
calculation. The PCPU address width would determine the VCPU address
width directly, and then the 64-bit PCI MMIO aperture would be
calculated from that.

However, there are two caveats.

(1) The larger your guest-phys address space (as exposed through the
VCPU address width to the rest of the firmware), the more guest RAM you
need for page tables. Because, just before entering the DXE phase, the
firmware builds 1:1 mapping page tables for the entire guest-phys
address space. This is necessary e.g. so you can access any PCI MMIO BAR.

Now consider that you have a huge beefy virtualization host with say 46
phys address bits, and a wimpy guest with say 1.5GB of guest RAM. Do you
absolutely want tens of *terabytes* for your 64-bit PCI MMIO aperture?
Do you really want to pay for the necessary page tables with that meager
guest RAM?

(Such machines do exist BTW, for example:

In other words, you'd need some kind of knob anyway, because otherwise
your aperture could grow too *large*.

(2) Exposing the PCPU address width to the guest may have nasty
consequences at the QEMU/KVM level, regardless of guest firmware. For
example, that kind of "guest enlightenment" could interfere with migration.

If you boot a guest let's say with 16GB of RAM, and tell it "hey friend,
have 40 bits of phys address width!", then you'll have a difficult time
migrating that guest to a host with a CPU that only has 36-bits wide
physical addresses -- even if the destination host has plenty of RAM
otherwise, such as a full 64GB.

There could be other QEMU/KVM / libvirt issues that I m unaware of
(hence the CC to Dave and Eduardo).
host physical address width gets messy. There are differences as well
between upstream qemu behaviour, and some downstreams.
I think the story is that:

a) Qemu default: 40 bits on any host
b) -cpu blah,host-phys-bits=true to follow the host.
c) RHEL has host-phys-bits=true by default

As you say, the only real problem with host-phys-bits is migration -
between say an E3 and an E5 xeon with different widths. The magic 40's
is generally wrong as well - I think it came from some ancient AMD,
but it's the default on QEMU TCG as well.

I don't think there's a way to set it in libvirt; is a bz asking for

IMHO host-phys-bits is actually pretty safe; and makes most sense in a
lot of cases.
Thanks -- this is a useful piece of the puzzle to know. It seems that
the guest can learn about the guest-phys address width via CPUID.
(cpu_x86_cpuid() in "target/i386/cpu.c" consumes "cpu->phys_bits", which
seems to be set in x86_cpu_realizefn().)


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