linux-kernel - Re: [PATCH] KVM: x86: Expose Predictive Store Forwarding Disable

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Message-ID: <fcbbdf83-128a-2519-13e8-1c5d5735a0d2@redhat.com>
Date:   Mon, 27 Sep 2021 13:13:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>, Babu Moger <babu.moger@....com>
Cc:     tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        hpa@...or.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tony.luck@...el.com, peterz@...radead.org,
        kyung.min.park@...el.com, wei.huang2@....com, jgross@...e.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Expose Predictive Store Forwarding Disable

On 27/09/21 12:59, Borislav Petkov wrote:
> On Thu, Sep 23, 2021 at 08:15:28PM -0500, Babu Moger wrote:
>> Linux kernel does not have the interface to enable/disable PSFD yet. Plan
>> here is to expose the PSFD technology to KVM so that the guest kernel can
>> make use of it if they wish to.
> 
> Why should the guest kernel expose it if we said that for now we want to
> disable it with the SSBD control?

Because the guest kernel needs to know which MSRs to write when you 
touch the SSBD prctl, so that PSFD is properly disabled *inside the guest*.

Paolo