Re: [PATCH v6 06/29] OvmfPkg/ResetVector: pre-validate the data pages used in SEC phase

Brijesh Singh

On 9/7/21 2:07 AM, wrote:

[ Looking at; ]

So, there isn't much tdx-specific in tdx-metadata. Most ranges are
TDX_METADATA_SECTION_TYPE_TEMP_MEM which I think basically means
these ranges should be accepted by the hypervisor, which is pretty much the
same issue snp tries to solve with this pre-validation range. Then there are
the ranges for code (aka bfv), for vars (aka cfv) and td_hob.

td_hob is the only tdx-specific item there, and even that concept (pass
memory ranges as hob list from hypervisor to guest) might be useful outside
Mailbox is tdx-specific too. But Stack/Heap/OvmfWorkarea/OvmfPageTable are
common. BFV/CFV are common too.
Mailbox is tagged "TDX_METADATA_SECTION_TYPE_TEMP_MEM", so nothing
special to do when loading the firmware, right?

I'd suggest we generalize the tdx-metadata idea and define both generic and
vmm-specific section types:

enum {

/* generic */
OVMF_SECTION_TYPE_SEC_MEM /* vmm should accept/validate this */

/* sev */
OVMF_SECTION_TYPE_SEV_CPUID /* or move to generic? */

/* tdx */

TDX has similar section type.
Yes. Both TDX and SNP have simliar requirements, they want store memory
ranges in the firmware binary in a way that allows qemu finding them and
using them when initializing the guest.
SNP stores the ranges directly in the GUID-chained block in the reset
vector. The range types are implicit (first is pre-validate area,
second is cpuid page, ...).
TDX stores a pointer to tdx-metadata in the GUID-chained block, then the
tdx-metadata has a list of ranges. The ranges are explicitly typed
(section type field).
The indirection used by TDX keeps the reset vector small. Also the
explicit typing of the ranges makes it easier to extend later on if
IMHO SEV should at minimum add explicit types to the memory ranges in
the boot block, but I'd very much prefer it if SEV and TDX can agree
on a way to store the memory ranges.

But I am not sure if SEV can use this metadata mechanism.
Need SEV's comments.
We should be able to make use of the metadata approach for the SEV-SNP. I will update the SNP patches to use the metadata approach in next rev.


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