amd/msr: allow passthrough of VIRT_SPEC_CTRL for HVM guests

Allow HVM guests access to MSR_VIRT_SPEC_CTRL if the platform Xen is
running on has support for it.  This requires adding logic in the
vm{entry,exit} paths for SVM in order to context switch between the
hypervisor value and the guest one.  The added handlers for context
switch will also be used for the legacy SSBD support.

Introduce a new synthetic feature leaf (X86_FEATURE_VIRT_SC_MSR_HVM)
to signal whether VIRT_SPEC_CTRL needs to be handled on guest
vm{entry,exit}.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/cpuid.c

@@ -541,6 +541,13 @@ static void __init calculate_hvm_max_policy(void)
          raw_cpuid_policy.basic.sep )
         __set_bit(X86_FEATURE_SEP, hvm_featureset);
 
+    /*
+     * VIRT_SSBD is exposed in the default policy as a result of
+     * VIRT_SC_MSR_HVM being set, it also needs exposing in the max policy.
+     */
+    if ( boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) )
+        __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
+
     /*
      * If Xen isn't virtualising MSR_SPEC_CTRL for HVM guests (functional
      * availability, or admin choice), hide the feature.
@@ -597,6 +604,13 @@ static void __init calculate_hvm_def_policy(void)
     guest_common_feature_adjustments(hvm_featureset);
     guest_common_default_feature_adjustments(hvm_featureset);
 
+    /*
+     * Only expose VIRT_SSBD if AMD_SSBD is not available, and thus
+     * VIRT_SC_MSR_HVM is set.
+     */
+    if ( boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) )
+        __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
+
     sanitise_featureset(hvm_featureset);
     cpuid_featureset_to_policy(hvm_featureset, p);
     recalculate_xstate(p);

xen/arch/x86/hvm/svm/entry.S

@@ -19,6 +19,8 @@
 
         .file "svm/entry.S"
 
+#include <xen/lib.h>
+
 #include <asm/asm_defns.h>
 #include <asm/page.h>
 
@@ -57,6 +59,9 @@ __UNLIKELY_END(nsvm_hap)
 
         clgi
 
+        ALTERNATIVE "", STR(call vmentry_virt_spec_ctrl), \
+                        X86_FEATURE_VIRT_SC_MSR_HVM
+
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
         /* SPEC_CTRL_EXIT_TO_SVM       Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
         .macro svm_vmentry_spec_ctrl
@@ -126,6 +131,9 @@ __UNLIKELY_END(nsvm_hap)
         ALTERNATIVE "", svm_vmexit_spec_ctrl, X86_FEATURE_SC_MSR_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
+        ALTERNATIVE "", STR(call vmexit_virt_spec_ctrl), \
+                        X86_FEATURE_VIRT_SC_MSR_HVM
+
         /*
          * STGI is executed unconditionally, and is sufficiently serialising
          * to safely resolve any Spectre-v1 concerns in the above logic.

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

@@ -52,6 +52,7 @@
 #include <asm/hvm/svm/svmdebug.h>
 #include <asm/hvm/svm/nestedsvm.h>
 #include <asm/hvm/nestedhvm.h>
+#include <asm/spec_ctrl.h>
 #include <asm/x86_emulate.h>
 #include <public/sched.h>
 #include <asm/hvm/vpt.h>
@@ -610,6 +611,16 @@ static void cf_check svm_cpuid_policy_changed(struct vcpu *v)
     svm_intercept_msr(v, MSR_SPEC_CTRL,
                       cp->extd.ibrs ? MSR_INTERCEPT_NONE : MSR_INTERCEPT_RW);
 
+    /*
+     * Always trap write accesses to VIRT_SPEC_CTRL in order to cache the guest
+     * setting and avoid having to perform a rdmsr on vmexit to get the guest
+     * setting even if VIRT_SSBD is offered to Xen itself.
+     */
+    svm_intercept_msr(v, MSR_VIRT_SPEC_CTRL,
+                      cp->extd.virt_ssbd && cpu_has_virt_ssbd &&
+                      !cpu_has_amd_ssbd ?
+                      MSR_INTERCEPT_WRITE : MSR_INTERCEPT_RW);
+
     /* Give access to MSR_PRED_CMD if the guest has been told about it. */
     svm_intercept_msr(v, MSR_PRED_CMD,
                       cp->extd.ibpb ? MSR_INTERCEPT_NONE : MSR_INTERCEPT_RW);
@@ -3105,6 +3116,30 @@ void svm_vmexit_handler(struct cpu_user_regs *regs)
     vmcb_set_vintr(vmcb, intr);
 }
 
+/* Called with GIF=0. */
+void vmexit_virt_spec_ctrl(void)
+{
+    unsigned int val = opt_ssbd ? SPEC_CTRL_SSBD : 0;
+
+    if ( val == current->arch.msrs->virt_spec_ctrl.raw )
+        return;
+
+    if ( cpu_has_virt_ssbd )
+        wrmsr(MSR_VIRT_SPEC_CTRL, val, 0);
+}
+
+/* Called with GIF=0. */
+void vmentry_virt_spec_ctrl(void)
+{
+    unsigned int val = current->arch.msrs->virt_spec_ctrl.raw;
+
+    if ( val == (opt_ssbd ? SPEC_CTRL_SSBD : 0) )
+        return;
+
+    if ( cpu_has_virt_ssbd )
+        wrmsr(MSR_VIRT_SPEC_CTRL, val, 0);
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/x86/include/asm/cpufeatures.h

@@ -24,7 +24,7 @@ XEN_CPUFEATURE(APERFMPERF,        X86_SYNTH( 8)) /* APERFMPERF */
 XEN_CPUFEATURE(MFENCE_RDTSC,      X86_SYNTH( 9)) /* MFENCE synchronizes RDTSC */
 XEN_CPUFEATURE(XEN_SMEP,          X86_SYNTH(10)) /* SMEP gets used by Xen itself */
 XEN_CPUFEATURE(XEN_SMAP,          X86_SYNTH(11)) /* SMAP gets used by Xen itself */
-/* Bit 12 - unused. */
+XEN_CPUFEATURE(VIRT_SC_MSR_HVM,   X86_SYNTH(12)) /* MSR_VIRT_SPEC_CTRL exposed to HVM */
 XEN_CPUFEATURE(IND_THUNK_LFENCE,  X86_SYNTH(13)) /* Use IND_THUNK_LFENCE */
 XEN_CPUFEATURE(IND_THUNK_JMP,     X86_SYNTH(14)) /* Use IND_THUNK_JMP */
 XEN_CPUFEATURE(SC_NO_BRANCH_HARDEN, X86_SYNTH(15)) /* (Disable) Conditional branch hardening */

xen/arch/x86/include/asm/msr.h

@@ -375,6 +375,16 @@ struct vcpu_msrs
      */
     uint32_t tsc_aux;
 
+    /*
+     * 0xc001011f - MSR_VIRT_SPEC_CTRL (if !X86_FEATURE_AMD_SSBD)
+     *
+     * AMD only. Guest selected value, context switched on guest VM
+     * entry/exit.
+     */
+    struct {
+        uint32_t raw;
+    } virt_spec_ctrl;
+
     /*
      * 0xc00110{27,19-1b} MSR_AMD64_DR{0-3}_ADDRESS_MASK
      *

xen/arch/x86/msr.c

@@ -393,7 +393,10 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
         if ( !cp->extd.virt_ssbd )
             goto gp_fault;
 
-        *val = msrs->spec_ctrl.raw & SPEC_CTRL_SSBD;
+        if ( cpu_has_amd_ssbd )
+            *val = msrs->spec_ctrl.raw & SPEC_CTRL_SSBD;
+        else
+            *val = msrs->virt_spec_ctrl.raw;
         break;
 
     case MSR_AMD64_DE_CFG:
@@ -686,10 +689,15 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
             goto gp_fault;
 
         /* Only supports SSBD bit, the rest are ignored. */
-        if ( val & SPEC_CTRL_SSBD )
-            msrs->spec_ctrl.raw |= SPEC_CTRL_SSBD;
+        if ( cpu_has_amd_ssbd )
+        {
+            if ( val & SPEC_CTRL_SSBD )
+                msrs->spec_ctrl.raw |= SPEC_CTRL_SSBD;
+            else
+                msrs->spec_ctrl.raw &= ~SPEC_CTRL_SSBD;
+        }
         else
-            msrs->spec_ctrl.raw &= ~SPEC_CTRL_SSBD;
+            msrs->virt_spec_ctrl.raw = val & SPEC_CTRL_SSBD;
         break;
 
     case MSR_AMD64_DE_CFG:

xen/arch/x86/spec_ctrl.c

@@ -513,9 +513,12 @@ static void __init print_details(enum ind_thunk thunk, uint64_t caps)
            (boot_cpu_has(X86_FEATURE_SC_MSR_HVM) ||
             boot_cpu_has(X86_FEATURE_SC_RSB_HVM) ||
             boot_cpu_has(X86_FEATURE_IBPB_ENTRY_HVM) ||
+            boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) ||
             opt_eager_fpu || opt_md_clear_hvm)       ? ""               : " None",
            boot_cpu_has(X86_FEATURE_SC_MSR_HVM)      ? " MSR_SPEC_CTRL" : "",
-           boot_cpu_has(X86_FEATURE_SC_MSR_HVM)      ? " MSR_VIRT_SPEC_CTRL" : "",
+           (boot_cpu_has(X86_FEATURE_SC_MSR_HVM) ||
+            boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM)) ? " MSR_VIRT_SPEC_CTRL"
+                                                       : "",
            boot_cpu_has(X86_FEATURE_SC_RSB_HVM)      ? " RSB"           : "",
            opt_eager_fpu                             ? " EAGER_FPU"     : "",
            opt_md_clear_hvm                          ? " MD_CLEAR"      : "",
@@ -1240,6 +1243,10 @@ void __init init_speculation_mitigations(void)
             setup_force_cpu_cap(X86_FEATURE_SC_MSR_HVM);
     }
 
+    /* Support VIRT_SPEC_CTRL.SSBD if AMD_SSBD is not available. */
+    if ( opt_msr_sc_hvm && !cpu_has_amd_ssbd && cpu_has_virt_ssbd )
+        setup_force_cpu_cap(X86_FEATURE_VIRT_SC_MSR_HVM);
+
     /* Figure out default_xen_spec_ctrl. */
     if ( has_spec_ctrl && ibrs )
     {