Re: [PATCH v4 2/2] KVM: SVM: Add support for Virtual SPEC_CTRL

[Jim Mattson]

On Thu, Jan 28, 2021 at 4:43 PM Babu Moger <babu.moger@xxxxxxx> wrote:

> This support also fixes an issue where a guest may sometimes see an
> inconsistent value for the SPEC_CTRL MSR on processors that support
> this feature. With the current SPEC_CTRL support, the first write to
> SPEC_CTRL is intercepted and the virtualized version of the SPEC_CTRL
> MSR is not updated. When the guest reads back the SPEC_CTRL MSR, it
> will be 0x0, instead of the actual expected value. There isn’t a
> security concern here, because the host SPEC_CTRL value is or’ed with
> the Guest SPEC_CTRL value to generate the effective SPEC_CTRL value.
> KVM writes with the guest's virtualized SPEC_CTRL value to SPEC_CTRL
> MSR just before the VMRUN, so it will always have the actual value
> even though it doesn’t appear that way in the guest. The guest will
> only see the proper value for the SPEC_CTRL register if the guest was
> to write to the SPEC_CTRL register again. With Virtual SPEC_CTRL
> support, the save area spec_ctrl is properly saved and restored.
> So, the guest will always see the proper value when it is read back.

Note that there are actually two significant problems with the way the
new feature interacts with the KVM code before this patch:
1) All bits set by the first non-zero write become sticky until the
vCPU is reset (because svm->spec_ctrl is never modified after the
first non-zero write).
2) The current guest IA32_SPEC_CTRL value isn't actually known to the
hypervisor. It thinks that there are no writes to the MSR after the
first non-zero write, so that sticky value will be returned to
KVM_GET_MSRS. This breaks live migration.

Basically, an always-on V_SPEC_CTRL breaks existing hypervisors. It
must, therefore, default to off. However, I see that our Rome and
Milan CPUs already report the existence of this feature.