[PATCH 4/5] x86/vmx: handle no model-specific LBR presence

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Sapphire Rapids have no model-specific LBRs, and instead only expose
architectural LBRs.  As documented in the Architectural Last Branch
Records specification, processors not supporting model-specific LBRs
MSR_IA32_DEBUGCTLMSR.LBR has no meaning, and can be written to 0 or 1,
but reads will always return 0.

Implement support in vmx_msr_write_intercept() by adding generic
detection of lack of model-specific LBRs by checking if the LBR format
reported in PERF_CAPABILITIES matches 0x3f, which is explicitly listed
in the manual as a way to signal lack of model-specific LBRs
presence.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Note the indentation change in vmx_msr_write_intercept() as a result
of the addition of a new condition is left for a following patch in
order to aid readability of the change.
---
 xen/arch/x86/hvm/vmx/vmx.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index cf428a4849..3f45ac05c6 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3007,6 +3007,8 @@ static const struct lbr_info {
     { MSR_GM_LASTBRANCH_0_FROM_IP,  NUM_MSR_GM_LASTBRANCH_FROM_TO },
     { MSR_GM_LASTBRANCH_0_TO_IP,    NUM_MSR_GM_LASTBRANCH_FROM_TO },
     { 0, 0 }
+}, no_lbr[] = {
+    {0, 0}
 };
 
 static const struct lbr_info *last_branch_msr_get(void)
@@ -3070,6 +3072,21 @@ static const struct lbr_info *last_branch_msr_get(void)
         /* Goldmont */
         case 0x5c: case 0x5f:
             return gm_lbr;
+
+        default:
+            if ( cpu_has_pdcm )
+            {
+                uint64_t cap;
+
+                rdmsrl(MSR_IA32_PERF_CAPABILITIES, cap);
+                if ( (cap & MSR_IA32_PERF_CAP_LBR_FORMAT) == 0x3f )
+                    /*
+                     * On processors that do not support model-specific LBRs,
+                     * PERF_CAPABILITIES.LBR_FMT will have the value 0x3f.
+                     */
+                    return no_lbr;
+            }
+            break;
         }
         break;
 
@@ -3521,6 +3538,8 @@ static int cf_check vmx_msr_write_intercept(
                 return X86EMUL_OKAY;
             }
 
+            if ( lbr->count )
+            {
             for ( ; lbr->count; lbr++ )
             {
                 unsigned int i;
@@ -3546,6 +3565,10 @@ static int cf_check vmx_msr_write_intercept(
                 v->arch.hvm.vmx.lbr_flags |= LBR_FIXUP_TSX;
             if ( ler_to_fixup_needed )
                 v->arch.hvm.vmx.lbr_flags |= LBR_FIXUP_LER_TO;
+            }
+            else
+                /* No model specific LBRs, ignore DEBUGCTLMSR.LBR. */
+                msr_content &= ~IA32_DEBUGCTLMSR_LBR;
         }
 
         __vmwrite(GUEST_IA32_DEBUGCTL, msr_content);
-- 
2.36.0