Re: [Xen-devel] [PATCH 8/9] x86/amd: Virtualise MSR_VIRT_SPEC_CTRL for guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.18 at 17:18, <andrew.cooper3@xxxxxxxxxx> wrote:
> The semantics of MSR_VIRT_SPEC_CTRL are that unknown bits are write-discard
> and read as zero.  Only VIRT_SPEC_CTRL.SSBD is defined at the moment.
> 
> To facilitate making this per-guest, the legacy SSBD state needs context
> switching between vcpus.  amd_ctxt_switch_legacy_ssbd() is updated to take the
> vcpus setting into account.  Furthermore, the guests chosen value needs
> preserving across migrate.
> 
> This marks a subtle change in how `ssbd=` behaves.  If Xen wishes SSBD to be
> asserted, it remains set in hardware all the time.  In the default case of Xen
> wishing SSBD not to be asserted, the value set in hardware is the guests
> choice.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>



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