Re: [PATCH Part1 v5 32/38] x86/sev: enable SEV-SNP-validated CPUID in #VC handlers

Brijesh Singh
2021-08-27T15:47:54Z

On 8/27/21 10:18 AM, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 10:19:27AM -0500, Brijesh Singh wrote:
>> From: Michael Roth <michael.roth@amd.com>
>>
>> This adds support for utilizing the SEV-SNP-validated CPUID table in
> s/This adds support for utilizing/Utilize/
>
> Yap, it can really be that simple. :)
>
>> the various #VC handler routines used throughout boot/run-time. Mostly
>> this is handled by re-using the CPUID lookup code introduced earlier
>> for the boot/compressed kernel, but at various stages of boot some work
>> needs to be done to ensure the CPUID table is set up and remains
>> accessible throughout. The following init routines are introduced to
>> handle this:
> Do not talk about what your patch does - that should hopefully be
> visible in the diff itself. Rather, talk about *why* you're doing what
> you're doing.
>
>> sev_snp_cpuid_init():
> This one is not really introduced - it is already there.
>
> <snip all the complex rest>
>
> So this patch is making my head spin. It seems we're dancing a lot of
> dance just to have our CPUID page present at all times. Which begs the
> question: do we need it during the whole lifetime of the guest?

Mike can correct me,  we need it for entire lifetime of the guest.
Whenever guest needs the CPUID value, the #VC handler will refer to this
page.

> Regardless, I think this can be simplified by orders of
> magnitude if we allocated statically 4K for that CPUID page in
> arch/x86/boot/compressed/mem_encrypt.S, copied the supplied CPUID page
> from the firmware to it and from now on, work with our own copy.

Actually a  VMM could populate more than one page for the CPUID. One
page can include 64 entries and I believe Mike is already running into
limits (with Qemu) and exploring the ideas to extend it more than a page.

> You probably would need to still remap it for kernel proper but it would
> get rid of all that crazy in this patch here.
>
> Hmmm?
>