linux-kernel - Re: [PATCH v2 1/1] KVM: x86: Propagate AMD-specific IBRS bits to guests

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Message-ID: <CALMp9eR766pb0664V3GHsq84qCCATRwUxPLrmeRUMv-6EYcs1Q@mail.gmail.com>
Date:   Fri, 24 Mar 2023 16:40:06 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Takahiro Itazuri <itazur@...zon.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, Takahiro Itazuri <zulinx86@...il.com>
Subject: Re: [PATCH v2 1/1] KVM: x86: Propagate AMD-specific IBRS bits to guests

On Fri, Mar 24, 2023 at 2:16 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Wed, Mar 01, 2023, Takahiro Itazuri wrote:
> > VMMs retrieve supported CPUID features via KVM_GET_SUPPORTED_CPUID to
> > construct CPUID information to be passed to KVM_SET_CPUID2. Most CPUID
> > feature bits related to speculative attacks are propagated from host
> > CPUID. AMD processors have AMD-specific IBRS related bits in CPUID
> > Fn8000_0008_EBX (ref: AMD64 Architecture Programmer's Manual Volume 3:
> > General-Purpose and System Instructions) and some bits are not
> > propagated to guests.
> >
> > Enable propagation of these bits to guests, so that guests can see the
> > same security information as the host without VMM action.
Usually, I can count on Sean for the semantic nitpick:

This propagates bits only to the userspace VMM. They may make it to
the guest. They may not.