nestedsvm: Support TSC Rate MSR

Support TSC Rate MSR and enable TSC scaling for
nested virtualization.

With it, guest VMs don't need take #VMEXIT to calculate a translated
TSC value when it is running under TSC emulation mode.

I measured native performance of the rdtsc instruction
in the l2 guest with xen-on-xen and both host and
and l1 guest run under TSC emulation mode.

TSC scaling just needs MSR emulation and correct tsc offset
calculation to be done and thus can be emulated also on older
hardware. In this case rdtsc instruction is intercepted and
handled by the host directly and safes the cost of a full
VMRUN/VMEXIT emulation cycle.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>

tools/libxc/xc_cpuid_x86.c

@@ -157,7 +157,7 @@ static void amd_xc_cpuid_policy(
             SVM_FEATURE_DECODEASSISTS);
 
         /* Pass 2: Always enable SVM features which are emulated */
-        regs[3] |= SVM_FEATURE_VMCBCLEAN;
+        regs[3] |= SVM_FEATURE_VMCBCLEAN | SVM_FEATURE_TSCRATEMSR;
         break;
     }
 

xen/arch/x86/hvm/svm/nestedsvm.c

@@ -147,6 +147,8 @@ int nsvm_vcpu_reset(struct vcpu *v)
     svm->ns_msr_hsavepa = VMCX_EADDR;
     svm->ns_ovvmcb_pa = VMCX_EADDR;
 
+    svm->ns_tscratio = DEFAULT_TSC_RATIO;
+
     svm->ns_cr_intercepts = 0;
     svm->ns_dr_intercepts = 0;
     svm->ns_exception_intercepts = 0;
@@ -1185,6 +1187,9 @@ int nsvm_rdmsr(struct vcpu *v, unsigned int msr, uint64_t *msr_content)
     case MSR_K8_VM_HSAVE_PA:
         *msr_content = svm->ns_msr_hsavepa;
         break;
+    case MSR_AMD64_TSC_RATIO:
+        *msr_content = svm->ns_tscratio;
+        break;
     default:
         ret = 0;
         break;
@@ -1211,6 +1216,16 @@ int nsvm_wrmsr(struct vcpu *v, unsigned int msr, uint64_t msr_content)
         }
         svm->ns_msr_hsavepa = msr_content;
         break;
+    case MSR_AMD64_TSC_RATIO:
+        if ((msr_content & ~TSC_RATIO_RSVD_BITS) != msr_content) {
+            gdprintk(XENLOG_ERR,
+                "reserved bits set in MSR_AMD64_TSC_RATIO 0x%"PRIx64"\n",
+                msr_content);
+            ret = -1; /* inject #GP */
+            break;
+        }
+        svm->ns_tscratio = msr_content;
+        break;
     default:
         ret = 0;
         break;

xen/arch/x86/hvm/svm/svm.c

@@ -635,28 +635,37 @@ static void svm_set_segment_register(struct vcpu *v, enum x86_segment seg,
         svm_vmload(vmcb);
 }
 
+static uint64_t svm_get_tsc_offset(uint64_t host_tsc, uint64_t guest_tsc,
+    uint64_t ratio)
+{
+    uint64_t offset;
+
+    if (ratio == DEFAULT_TSC_RATIO)
+        return guest_tsc - host_tsc;
+
+    /* calculate hi,lo parts in 64bits to prevent overflow */
+    offset = (((host_tsc >> 32U) * (ratio >> 32U)) << 32U) +
+          (host_tsc & 0xffffffffULL) * (ratio & 0xffffffffULL);
+    return guest_tsc - offset;
+}
+
 static void svm_set_tsc_offset(struct vcpu *v, u64 offset)
 {
     struct vmcb_struct *vmcb = v->arch.hvm_svm.vmcb;
     struct vmcb_struct *n1vmcb, *n2vmcb;
     uint64_t n2_tsc_offset = 0;
     struct domain *d = v->domain;
+    uint64_t host_tsc, guest_tsc;
+
+    guest_tsc = hvm_get_guest_tsc(v);
+
+    /* Re-adjust the offset value when TSC_RATIO is available */
+    if ( cpu_has_tsc_ratio && d->arch.vtsc ) {
+        rdtscll(host_tsc);
+        offset = svm_get_tsc_offset(host_tsc, guest_tsc, vcpu_tsc_ratio(v));
+    }
 
     if ( !nestedhvm_enabled(d) ) {
-        /* Re-adjust the offset value when TSC_RATIO is available */
-        if ( cpu_has_tsc_ratio && d->arch.vtsc )
-        {
-            uint64_t host_tsc, guest_tsc;
-
-            rdtscll(host_tsc);
-            guest_tsc = hvm_get_guest_tsc(v);
-            
-            /* calculate hi,lo parts in 64bits to prevent overflow */
-            offset = (((host_tsc >> 32) * d->arch.tsc_khz / cpu_khz) << 32) +
-                     (host_tsc & 0xffffffffULL) * d->arch.tsc_khz / cpu_khz;
-            offset = guest_tsc - offset;
-        }
-
         vmcb_set_tsc_offset(vmcb, offset);
         return;
     }
@@ -665,8 +674,14 @@ static void svm_set_tsc_offset(struct vcpu *v, u64 offset)
     n2vmcb = vcpu_nestedhvm(v).nv_n2vmcx;
 
     if ( nestedhvm_vcpu_in_guestmode(v) ) {
+        struct nestedsvm *svm = &vcpu_nestedsvm(v);
+
         n2_tsc_offset = vmcb_get_tsc_offset(n2vmcb) -
             vmcb_get_tsc_offset(n1vmcb);
+        if ( svm->ns_tscratio != DEFAULT_TSC_RATIO ) {
+            n2_tsc_offset = svm_get_tsc_offset(guest_tsc,
+                guest_tsc + n2_tsc_offset, svm->ns_tscratio);
+        }
         vmcb_set_tsc_offset(n1vmcb, offset);
     }
 
@@ -1107,6 +1122,7 @@ struct hvm_function_table * __init start_svm(void)
     P(cpu_has_svm_cleanbits, "VMCB Clean Bits");
     P(cpu_has_svm_decode, "DecodeAssists");
     P(cpu_has_pause_filter, "Pause-Intercept Filter");
+    P(cpu_has_tsc_ratio, "TSC Rate MSR");
 #undef P
 
     if ( !printed )

xen/include/asm-x86/hvm/svm/nestedsvm.h

@@ -36,6 +36,11 @@ struct nestedsvm {
      */
     uint64_t ns_ovvmcb_pa;
 
+    /* virtual tscratio holding the value l1 guest writes to the
+     * MSR_AMD64_TSC_RATIO MSR.
+     */
+    uint64_t ns_tscratio;
+
     /* Cached real intercepts of the l2 guest */
     uint32_t ns_cr_intercepts;
     uint32_t ns_dr_intercepts;