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

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Date:   Tue, 18 Jan 2022 08:37:30 -0600
From:   Michael Roth <michael.roth@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Brijesh Singh <brijesh.singh@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <linux-efi@...r.kernel.org>, <platform-driver-x86@...r.kernel.org>,
        <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        <tony.luck@...el.com>, <marcorr@...gle.com>,
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH v8 29/40] x86/compressed/64: add support for SEV-SNP
 CPUID table in #VC handlers

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.