linux-kernel - Re: [PATCH 05/23] KVM: x86: Add SBPB support

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Message-ID: <ZOjw4uPOKltexXOp@google.com>
Date:   Fri, 25 Aug 2023 11:20:18 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>, David.Kaplan@....com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 05/23] KVM: x86: Add SBPB support

On Fri, Aug 25, 2023, Josh Poimboeuf wrote:
> Add support for the AMD Selective Branch Predictor Barrier (SBPB) by
> advertising the CPUID bit and handling PRED_CMD writes accordingly.

Same as the other patch, please call out that not doing the "standard" F(SBPB)
is intentional, e.g.

  Note, like SRSO_NO and IBPB_BRTYPE before it, advertise support for SBPB
  even if it's not enumerated by in the raw CPUID.  Some CPUs that gained
  support via a uCode patch don't report SBPB via CPUID (the kernel forces
  the flag).

And again, feel free to take this through tip if this should go in 6.6.  Turns out
our Milan systems have the SBPB fun, so I was able to actually test this, including
the emulated WRMSR handling (KVM allows forcing emulation via a magic prefix).  I
have a KVM-Unit-Test patch that I'll post next week.

Thanks Josh!

Acked-by: Sean Christopherson <seanjc@...gle.com>

> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>