[PATCH v1] x86/nSVM: Expose the FlushByASID CPU Capability to L1 guests

[Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>]

On the AMD platforms, the Xen hypervisor requires the FlushByASID CPU
capability to support HVM nested virtualization (see start_nested_svm).
Consequently, the L1 hypervisor must report FlushByASID CPU capability support
when intercepting CPUID instruction for the CPU feature from the L2 guest to
support nested virtualization levels beyond L1. Extend the exposed HVM CPU
policy to surface this CPU feature support for the guests.

While at it remove the dangling `exitinfo1 = ns_vmcb->exitinfo1;` assignment
inside the nested exit handling of svm_vmexit_handler().

Signed-off-by: Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>
---
Testing:
 - Using a locally developed XTF test to call cpuid_edx(0x8000000aU);
   - without the change, EDX returns 0x4AB (the FlushByASID 6th bit is not
     set).
   - with the change, EDX returns 0x4EB (the FlushByASID 6th bit set).
 - CI tests:
https://gitlab.com/xen-project/people/aabdelsa/xen/-/pipelines/2757859821
---
 xen/arch/x86/cpu-policy.c  | 1 +
 xen/arch/x86/hvm/svm/svm.c | 1 -
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index eddcd9778f..48a3185eed 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -843,6 +843,7 @@ static void __init calculate_hvm_max_policy(void)
         p->extd.raw[0xa].d &= ((1u << SVM_FEATURE_NPT) |
                                (1u << SVM_FEATURE_LBRV) |
                                (1u << SVM_FEATURE_NRIPS) |
+                               (1u << SVM_FEATURE_FLUSHBYASID) |
                                (1u << SVM_FEATURE_PAUSEFILTER) |
                                (1u << SVM_FEATURE_DECODEASSISTS));
         /* Enable features which are always emulated. */
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 38c61db1d7..2f62981305 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2561,7 +2561,6 @@ void asmlinkage svm_vmexit_handler(void)
          * nestedsvm_check_intercepts() expects to have the correct
          * exitinfo1 value there.
          */
-        exitinfo1 = ns_vmcb->exitinfo1;
         ns_vmcb->exitinfo1 = vmcb->exitinfo1;
         nsret = nestedsvm_check_intercepts(v, regs, exit_reason);
         switch ( nsret )
-- 
2.53.0