[Xen-devel] [PATCH v10 02/11] x86/msr: Emulation of MSR_{SPEC_CTRL, PRED_CMD} for guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

As per the spec currently available here:

https://software.intel.com/sites/default/files/managed/c5/63/336996-Speculative-Execution-Side-Channel-Mitigations.pdf

MSR_ARCH_CAPABILITIES will only come into existence on new hardware, but is
implemented as a straight #GP for now to avoid being leaky when new hardware
arrives.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>

v9:
 * Alter the STIBP handling to match Intel's latest spec
 * Drop spec_ctrl.guest as it is no longer needed
---
 xen/arch/x86/msr.c              | 45 +++++++++++++++++++++++++++++++++++++++++
 xen/include/asm-x86/msr-index.h |  2 ++
 xen/include/asm-x86/msr.h       | 10 +++++++++
 3 files changed, 57 insertions(+)

diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 187f862..7875d9c 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -120,11 +120,22 @@ int init_vcpu_msr_policy(struct vcpu *v)
 
 int guest_rdmsr(const struct vcpu *v, uint32_t msr, uint64_t *val)
 {
+    const struct cpuid_policy *cp = v->domain->arch.cpuid;
     const struct msr_domain_policy *dp = v->domain->arch.msr;
     const struct msr_vcpu_policy *vp = v->arch.msr;
 
     switch ( msr )
     {
+    case MSR_PRED_CMD:
+        /* Write-only */
+        goto gp_fault;
+
+    case MSR_SPEC_CTRL:
+        if ( !cp->feat.ibrsb )
+            goto gp_fault;
+        *val = vp->spec_ctrl.raw;
+        break;
+
     case MSR_INTEL_PLATFORM_INFO:
         if ( !dp->plaform_info.available )
             goto gp_fault;
@@ -132,6 +143,10 @@ int guest_rdmsr(const struct vcpu *v, uint32_t msr, 
uint64_t *val)
                _MSR_PLATFORM_INFO_CPUID_FAULTING;
         break;
 
+    case MSR_ARCH_CAPABILITIES:
+        /* Not implemented yet. */
+        goto gp_fault;
+
     case MSR_INTEL_MISC_FEATURES_ENABLES:
         if ( !vp->misc_features_enables.available )
             goto gp_fault;
@@ -153,14 +168,44 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t 
val)
 {
     const struct vcpu *curr = current;
     struct domain *d = v->domain;
+    const struct cpuid_policy *cp = d->arch.cpuid;
     struct msr_domain_policy *dp = d->arch.msr;
     struct msr_vcpu_policy *vp = v->arch.msr;
 
     switch ( msr )
     {
     case MSR_INTEL_PLATFORM_INFO:
+    case MSR_ARCH_CAPABILITIES:
+        /* Read-only */
         goto gp_fault;
 
+    case MSR_SPEC_CTRL:
+        if ( !cp->feat.ibrsb )
+            goto gp_fault; /* MSR available? */
+
+        /*
+         * Note: SPEC_CTRL_STIBP is specified as safe to use (i.e. ignored)
+         * when STIBP isn't enumerated in hardware.
+         */
+
+        if ( val & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP) )
+            goto gp_fault; /* Rsvd bit set? */
+
+        vp->spec_ctrl.raw = val;
+        break;
+
+    case MSR_PRED_CMD:
+        if ( !cp->feat.ibrsb && !cp->extd.ibpb )
+            goto gp_fault; /* MSR available? */
+
+        /*
+         * The only defined behaviour is when writing PRED_CMD_IBPB.  In
+         * practice, real hardware accepts any value without faulting.
+         */
+        if ( v == curr && (val & PRED_CMD_IBPB) )
+            wrmsrl(MSR_PRED_CMD, PRED_CMD_IBPB);
+        break;
+
     case MSR_INTEL_MISC_FEATURES_ENABLES:
     {
         uint64_t rsvd = ~0ull;
diff --git a/xen/include/asm-x86/msr-index.h b/xen/include/asm-x86/msr-index.h
index a0aacfa..23ad743 100644
--- a/xen/include/asm-x86/msr-index.h
+++ b/xen/include/asm-x86/msr-index.h
@@ -39,6 +39,8 @@
 #define MSR_PRED_CMD                   0x00000049
 #define PRED_CMD_IBPB                  (_AC(1, ULL) << 0)
 
+#define MSR_ARCH_CAPABILITIES          0x0000010a
+
 /* Intel MSRs. Some also available on other CPUs */
 #define MSR_IA32_PERFCTR0              0x000000c1
 #define MSR_IA32_A_PERFCTR0            0x000004c1
diff --git a/xen/include/asm-x86/msr.h b/xen/include/asm-x86/msr.h
index 2fbed02..928f1cc 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -223,6 +223,16 @@ struct msr_domain_policy
 /* MSR policy object for per-vCPU MSRs */
 struct msr_vcpu_policy
 {
+    /* 0x00000048 - MSR_SPEC_CTRL */
+    struct {
+        /*
+         * Only the bottom two bits are defined, so no need to waste space
+         * with uint64_t at the moment, but use uint32_t for the convenience
+         * of the assembly code.
+         */
+        uint32_t raw;
+    } spec_ctrl;
+
     /* 0x00000140  MSR_INTEL_MISC_FEATURES_ENABLES */
     struct {
         bool available; /* This MSR is non-architectural */
-- 
2.1.4


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