[Xen-devel] [PATCH v6.5 17/26] x86/msr: Emulation of MSR_{SPEC_CTRL, PRED_CMD} for guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3:
 * Brackets around && (.. & ..) operation
 * Extend host to uint32_t for the benefit of the asm code.
v4:
 * Extend MSR_PRED_CMD availability logic for AMD.
---
 xen/arch/x86/msr.c        | 35 +++++++++++++++++++++++++++++++++++
 xen/include/asm-x86/msr.h | 12 ++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 31983ed..02a7b49 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -119,11 +119,22 @@ 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.guest;
+        break;
+
     case MSR_INTEL_PLATFORM_INFO:
         if ( !dp->plaform_info.available )
             goto gp_fault;
@@ -152,14 +163,38 @@ 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:
+        /* Read-only */
         goto gp_fault;
 
+    case MSR_SPEC_CTRL:
+        if ( !cp->feat.ibrsb )
+            goto gp_fault; /* MSR available? */
+        if ( val & ~(SPEC_CTRL_IBRS |
+                     (cp->feat.stibp ? SPEC_CTRL_STIBP : 0)) )
+            goto gp_fault; /* Rsvd bit set? */
+        vp->spec_ctrl.guest = val;
+        vp->spec_ctrl.host  = 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.h b/xen/include/asm-x86/msr.h
index 2fbed02..3d0012d 100644
--- a/xen/include/asm-x86/msr.h
+++ b/xen/include/asm-x86/msr.h
@@ -223,6 +223,18 @@ 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.  We maintain the guests idea of the
+         * value it wrote, and a value to install into hardware (extended to
+         * uint32_t to simplify the asm) which might be different.
+         */
+        uint32_t host;
+        uint8_t guest;
+    } spec_ctrl;
+
     /* 0x00000140  MSR_INTEL_MISC_FEATURES_ENABLES */
     struct {
         bool available; /* This MSR is non-architectural */
-- 
2.1.4


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