linux-kernel - Re: [PATCH kernel 3/3] x86/sev: Do not handle #VC for DR7 read/write

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Date:   Thu, 8 Dec 2022 12:01:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Alexey Kardashevskiy <aik@....com>
Cc:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Venu Busireddy <venu.busireddy@...cle.com>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Michael Sterritt <sterritt@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Mario Limonciello <mario.limonciello@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH kernel 3/3] x86/sev: Do not handle #VC for DR7 read/write

On Thu, Dec 08, 2022 at 06:14:01PM +1100, Alexey Kardashevskiy wrote:
> Sorry, I am not following. How is moving the bit makes
> SEV_STATUS_TO_FEATURES() not needed?
> 
> When I am setting it in VMSA SEV_FEATURES - it is a bit 5.
> 
> Inside a SEV VM, it is SEV_STATUS MSR and there it is bit 7. Mentioned
> MSR_AMD64_SEV* are SEV_STATUS MSR bits.
> 
> Since the current patch is bad, I'd rather define the bit twice then:

Yes.

> arch/x86/include/asm/msr-index.h:
> #define MSR_AMD64_SEV_FEAT_DEBUG_SWAP    BIT_ULL(7)
> 
> arch/x86/include/asm/svm.h
> #define SVM_SEV_FEAT_DEBUG_SWAP         BIT(5)
> 
> as nothing really says that SEV_FEATURES is always going to be
> SEV_STATUS>>2, even though it is now.
> 
> Soooo what is acceptable solution here? Thanks,

Right, so since you're testing against sev_status which is a copy of
MSR_AMD64_SEV, then you use bit definitions which are for that MSR as
documented in the respective PPR section for "MSRC001_0131 [SEV Status]
(Core::X86::Msr::SEV_Status)"

When you're setting the VMSA's SEV_FEATURES field, then you need a
different define, ofc.

This also automatically takes care of SEV_FEATURES not being tied to
SEV_STATUS >> 2 forever, as you say.

So yes, do the twice thing.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette