Re: [PATCH 8/8] x86/cpuid: Enable MSR_SPEC_CTRL in SVM guests by default

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 26/01/2022 08:44, Andrew Cooper wrote:
> With all other pieces in place, MSR_SPEC_CTRL is fully working for HVM guests.
>
> Update the CPUID derivation logic (both PV and HVM to avoid losing subtle
> changes), and explicitly enable the CPUID bits for HVM guests.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

SVM guests get rather more speedy with this hunk, which I missed:

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index bc834556c5f7..f11622ed4ff8 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -606,6 +606,10 @@ static void svm_cpuid_policy_changed(struct vcpu *v)
 
     vmcb_set_exception_intercepts(vmcb, bitmap);
 
+    /* Give access to MSR_SPEC_CTRL if the guest has been told about it. */
+    svm_intercept_msr(v, MSR_SPEC_CTRL,
+                      cp->extd.ibrs ? MSR_INTERCEPT_NONE :
MSR_INTERCEPT_RW);
+
     /* Give access to MSR_PRED_CMD if the guest has been told about it. */
     svm_intercept_msr(v, MSR_PRED_CMD,
                       cp->extd.ibpb ? MSR_INTERCEPT_NONE :
MSR_INTERCEPT_RW);


I've folded it into v2, but won't repost for just this.

~Andrew