linux-kernel - Re: [PATCH] kvm: x86: Report STIBP on GET_SUPPORTED

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Date:   Fri, 7 Dec 2018 14:48:37 -0200
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        KarimAllah Ahmed <karahmed@...zon.de>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] kvm: x86: Report STIBP on GET_SUPPORTED_CPUID

On Wed, Dec 05, 2018 at 05:02:06PM -0500, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 05, 2018 at 05:19:56PM -0200, Eduardo Habkost wrote:
> > Months ago, we have added code to allow direct access to MSR_IA32_SPEC_CTRL
> > to the guest, which makes STIBP available to guests.  This was implemented
> > by commits d28b387fb74d ("KVM/VMX: Allow direct access to
> > MSR_IA32_SPEC_CTRL") and b2ac58f90540 ("KVM/SVM: Allow direct access to
> > MSR_IA32_SPEC_CTRL").
> > 
> > However, we never updated GET_SUPPORTED_CPUID to let userspace know that
> > STIBP can be enabled in CPUID.  Fix that by updating
> 
> Ooops!
> > kvm_cpuid_8000_0008_ebx_x86_features and kvm_cpuid_7_0_edx_x86_features.
> 
> Shouldn't there also be a patch in QEMU to use it? (aka, +stibp).

I will submit the QEMU patch soon.

A patch exists on some downstream QEMU distributions, already,
but it was never merged upstream because GET_SUPPORTED_CPUID
never supported STIBP in the upstream kernel.  (And because in
the end it was not used for mitigating Spectre)

> 
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

Thanks!

-- 
Eduardo