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

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Message-ID: <62e8b7f2-4e0d-5836-ea37-9e0a7a797017@amd.com>
Date:   Fri, 27 Aug 2021 10:47:42 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     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>,
        Wanpeng Li <wanpengli@...cent.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>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 32/38] x86/sev: enable SEV-SNP-validated CPUID in
 #VC handlers


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@....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?
>