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

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Date:   Mon, 27 Sep 2021 10:47:01 -0500
From:   Babu Moger <babu.moger@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>
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 9/27/21 7:54 AM, Paolo Bonzini wrote:
> On 27/09/21 14:28, Borislav Petkov wrote:
>> On Mon, Sep 27, 2021 at 02:14:52PM +0200, Paolo Bonzini wrote:
>>> Right, not which MSR to write but which value to write.  It doesn't know
>>> that the PSF disable bit is valid unless the corresponding CPUID bit is
>>> set.
>>
>> There's no need for the separate PSF CPUID bit yet. We have decided for
>> now to not control PSF separately but disable it through SSB. Please
>> follow this thread:
> 
> There are other guests than Linux.  This patch is just telling userspace

Yes, That is the reason for this patch.

> that KVM knows what the PSFD bit is.  It is also possible to expose the
> bit in KVM without having any #define in cpufeatures.h or without the
> kernel using it.  For example KVM had been exposing FSGSBASE long before
> Linux supported it.
> 
> That said, the patch is incomplete because it should also add the new
> CPUID bit to guest_has_spec_ctrl_msr (what KVM *really* cares about is not
> the individual bits, only whether SPEC_CTRL exists at all).

Yea, I missed that. Will add it in next revision,
Thanks
Babu