linux-kernel - Re: [PATCH 2/5] KVM: x86: do not modify masked bits of shared MSRs

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Message-ID: <CALMp9eTzAFYt1wkXT+OEx=vNs0rrCvp=8XG8Jbwwaj3mSPPF+Q@mail.gmail.com>
Date:   Tue, 19 Nov 2019 11:00:28 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/5] KVM: x86: do not modify masked bits of shared MSRs

On Mon, Nov 18, 2019 at 10:17 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> "Shared MSRs" are guest MSRs that are written to the host MSRs but
> keep their value until the next return to userspace.  They support
> a mask, so that some bits keep the host value, but this mask is
> only used to skip an unnecessary MSR write and the value written
> to the MSR is always the guest MSR.
>
> Fix this and, while at it, do not update smsr->values[slot].curr if
> for whatever reason the wrmsr fails.  This should only happen due to
> reserved bits, so the value written to smsr->values[slot].curr
> will not match when the user-return notifier and the host value will
> always be restored.  However, it is untidy and in rare cases this
> can actually avoid spurious WRMSRs on return to userspace.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>