Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP CPUID table in #VC handlers

From: Michael Roth <michael.roth@amd.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Brijesh Singh <brijesh.singh@amd.com>, <x86@kernel.org>,
	<linux-kernel@vger.kernel.org>, <kvm@vger.kernel.org>,
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	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
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	"H. Peter Anvin" <hpa@zytor.com>,
	Ard Biesheuvel <ardb@kernel.org>,
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	Sean Christopherson <seanjc@google.com>,
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	<sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP CPUID table in #VC handlers
Date: Tue, 18 Jan 2022 08:23:45 -0600	[thread overview]
Message-ID: <20220118142345.65wuub2p3alavhpb@amd.com> (raw)
In-Reply-To: <YebIiN6Ftq2aPtyF@zn.tnic>

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
it slightly that so snp_cpuid_info_get_ptr() does the same check that
snp_cpuid_active() does, and have it return NULL if the copy hasn't been
initialized, if that seems preferable to the separate snp_cpuid_active()
function.

> 
> ---
> 
> Btw, all those
> 
>         /* SEV-SNP CPUID table should be set up now. */
>         if (!snp_cpuid_active())
>                 sev_es_terminate(1, GHCB_TERM_CPUID);
> 
> after snp_cpuid_info_create() has returned are useless either. If that
> function returns, you know you're good to go wrt SNP.

It seemed like a good thing to assert in case something slipped in later
that tried to use snp_cpuid() without the table being initialized, but
if I implement things the way you suggested above,
snp_cpuid_info_get_ptr() will return NULL in that case, so we get that
assurance for free. That does sound cleaner.

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