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

[Babu Moger]

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