Re: [PATCH v8 01/40] x86/compressed/64: detect/setup SEV/SME features earlier in boot

Michael Roth
2021-12-15T20:19:31Z

On Wed, Dec 15, 2021 at 07:33:47PM +0100, Borislav Petkov wrote:
> On Wed, Dec 15, 2021 at 12:17:44PM -0600, Venu Busireddy wrote:
> > Boris & Tom, which implementation would you prefer?
>
> I'd like to see how that sme_sev_parse_cpuid() would look like. And that
> function should be called sev_parse_cpuid(), btw.
>
> Because if that function turns out to be a subset of your suggestion,
> functionality-wise, then we should save us the churn and simply do one
> generic helper.

I was actually thinking this proposed sev_parse_cpuid() helper would be
a superset of what Venu currently has implemented. E.g. Venu's most recent
patch does:

sev_enable():
unsigned int me_bit_pos;

me_bit_pos = get_me_bit(AMD_SEV_BIT)
if (!me_bit_pos)
return;

...

Let's say in the future there's need to also grab say, the VTE bit. We
could introduce a new helper, get_vte_bit() that re-does all the
0x80000000-0x8000001F range checks, some sanity checks that SEV is set if
VTE bit is set, and then now have a nice single-purpose helper that
duplicates similar checks in get_me_bit(), or we could avoid the
duplication by expanding get_me_bit() so it could be used something like:

me_bit_pos = get_me_bit(AMD_SEV_BIT, &vte_enabled)

at which point it makes more sense to just have it be a more generic
helper, called via:

ret = sev_parse_cpuid(AMD_SEV_BIT, &me_bit_pos, &vte_enabled)

i.e. Venu's original patch basically, but with the helper function
renamed.

and if fields are added in the future:

sev_parse_cpuid(AMD_SEV_BIT, &me_bit_pos, &vte_enabled, &new_feature_enabled, etc..)

or if that eventually becomes unwieldly it could later be changed to return
a feature mask.

>
> Btw 2, that helper should be in arch/x86/kernel/sev-shared.c so that it
> gets shared by both kernel stages instead having an inline function in
> some random header.
>
> Btw 3, I'm not crazy about the feature testing with the @features param
> either. Maybe that function should return the eYx register directly,
> like the cpuid_eYx() variants in the kernel do, where Y in { a, b, c, d
> }.
>
> The caller can than do its own testing:
>
> eax = sev_parse_cpuid(RET_EAX, ...)
> if (eax > 0) {
> if (eax & BIT(1))
> ...
>
> Something along those lines, for example.

I think having sev_parse_cpuid() using a more "human-readable" format
for reporting features/fields will make it easier to abstract away the
nitty-gritty details and reduce that chances for more duplication
between boot/compressed and kernel proper in the future. That
"human-readable" format could be in the form of a boolean/int
parameter list that gets expanded over time as needed (like the above
examples), or a higher-level construct like a struct/bitmask/etc. But
either way it would be nice to only have to think about specific CPUID
bits when looking at sev_parse_cpuid(), and have callers instead rely
purely on the sev_parse_cpuid() function prototype/documentation to
know what's going on.

>
> But I'd have to see a concrete diff from Michael to get a better idea
> how that CPUID parsing from the CPUID page is going to look like.

It should look the same with/without CPUID page, since the CPUID page
will have already been set up early in sev_enable()/sme_enable() based
on the presence of the CC blob via snp_init(), introduced in:

[PATCH v8 31/40] x86/compressed: add SEV-SNP feature detection/setup

Thanks,

Mike

>
> Thx.
>
> --
> Regards/Gruss,
> Boris.
>
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