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Date:   Tue, 18 Jan 2022 08:37:30 -0600
From:   Michael Roth <michael.roth@amd.com>
To:     Borislav Petkov <bp@alien8.de>
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Subject: Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP
 CPUID table in #VC handlers
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On Tue, Jan 18, 2022 at 08:32:38AM -0600, Michael Roth wrote:
> On Tue, Jan 18, 2022 at 08:23:45AM -0600, Michael Roth wrote:
> > On Tue, Jan 18, 2022 at 03:02:48PM +0100, Borislav Petkov wrote:
> > > On Mon, Jan 17, 2022 at 10:35:21PM -0600, Michael Roth wrote:
> > > > Unfortunately, in sev_enable(), between the point where snp_init() is
> > > > called, and sev_status is actually set, there are a number of cpuid
> > > > intructions which will make use of do_vc_no_ghcb() prior to sev_status
> > > > being set (and it needs to happen in that order to set sev_status
> > > > appropriately). After that point, snp_cpuid_active() would no longer be
> > > > necessary, but during that span some indicator is needed in case this
> > > > is just an SEV-ES guest trigger cpuid #VCs.
> > > 
> > > You mean testing what snp_cpuid_info_create() set up is not enough?
> > > 
> > > diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> > > index 7bc7e297f88c..17cfe804bad3 100644
> > > --- a/arch/x86/kernel/sev-shared.c
> > > +++ b/arch/x86/kernel/sev-shared.c
> > > @@ -523,7 +523,9 @@ static int snp_cpuid_postprocess(u32 func, u32 subfunc, u32 *eax, u32 *ebx,
> > >  static int snp_cpuid(u32 func, u32 subfunc, u32 *eax, u32 *ebx, u32 *ecx,
> > >  		     u32 *edx)
> > >  {
> > > -	if (!snp_cpuid_active())
> > > +	const struct snp_cpuid_info *c = snp_cpuid_info_get_ptr();
> > > +
> > > +	if (!c->count)
> > >  		return -EOPNOTSUPP;
> > >  
> > >  	if (!snp_cpuid_find_validated_func(func, subfunc, eax, ebx, ecx, edx)) {
> > 
> > snp_cpuid_info_get_ptr() will always return non-NULL, since it's a
> > pointer to the local copy of the cpuid page. But I can probably re-work
> 
> Doh, misread your patch. Yes I think checking the count would also work,
> since a valid table should be non-zero.

Actually, no, because doing that would provide hypervisor a means to
effectively disable CPUID page for an SNP guest by provided a table with
count == 0, which needs to be guarded against.

But can still implement something similar by having snp_cpuid_info_get_ptr()
return NULL if local copy of cpuid page hasn't been initialized.