[PATCH 2/3] amd/msr: allow passthrough of VIRT_SPEC_CTRL for HVM guests

[Roger Pau Monne <roger.pau@citrix.com>]

Allow HVM guests untrapped access to MSR_VIRT_SPEC_CTRL if the
hardware 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.

Note that the implementation relies on storing the guest value in the
spec_ctrl MSR per-vCPU variable, as the usage of VIRT_SPEC_CTRL
precludes the usage of SPEC_CTRL. Also store the current and
hypervisor states of VIRT_SPEC_CTRL in the per-pCPU spec_ctrl fields
at cpu_info in order to properly context switch the values between
Xen and HVM guests.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/arch/x86/cpu/amd.c                 |  7 +++-
 xen/arch/x86/cpuid.c                   | 11 ++++++
 xen/arch/x86/hvm/svm/entry.S           |  8 +++-
 xen/arch/x86/hvm/svm/svm.c             | 55 ++++++++++++++++++++++++++
 xen/arch/x86/include/asm/cpufeatures.h |  1 +
 xen/arch/x86/spec_ctrl.c               |  8 +++-
 6 files changed, 86 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index a8e37dbb1f..c3fcc0e558 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -687,6 +687,7 @@ void amd_init_lfence(struct cpuinfo_x86 *c)
  */
 void amd_init_ssbd(const struct cpuinfo_x86 *c)
 {
+	struct cpu_info *info = get_cpu_info();
 	int bit = -1;
 
 	if (cpu_has_ssb_no)
@@ -699,7 +700,7 @@ void amd_init_ssbd(const struct cpuinfo_x86 *c)
 
 	if (cpu_has_virt_ssbd) {
 		wrmsrl(MSR_VIRT_SPEC_CTRL, opt_ssbd ? SPEC_CTRL_SSBD : 0);
-		return;
+		goto out;
 	}
 
 	switch (c->x86) {
@@ -729,6 +730,10 @@ void amd_init_ssbd(const struct cpuinfo_x86 *c)
 
 	if (bit < 0)
 		printk_once(XENLOG_ERR "No SSBD controls available\n");
+
+ out:
+	info->last_spec_ctrl = info->xen_spec_ctrl = opt_ssbd ? SPEC_CTRL_SSBD
+							      : 0;
 }
 
 void __init detect_zen2_null_seg_behaviour(void)
diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 29b4cfc9e6..7b10fbf12f 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -551,6 +551,9 @@ static void __init calculate_hvm_max_policy(void)
          */
         __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
 
+    if ( boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) )
+        __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
+
     /*
      * With VT-x, some features are only supported by Xen if dedicated
      * hardware support is also available.
@@ -590,6 +593,14 @@ static void __init calculate_hvm_def_policy(void)
     guest_common_feature_adjustments(hvm_featureset);
     guest_common_default_feature_adjustments(hvm_featureset);
 
+    /*
+     * Only expose VIRT_SPEC_CTRL support by default if SPEC_CTRL is not
+     * supported.
+     */
+    if ( boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) &&
+         !boot_cpu_has(X86_FEATURE_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);
diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index 4ae55a2ef6..2a0c41625b 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -71,7 +71,9 @@ __UNLIKELY_END(nsvm_hap)
             mov    %al, CPUINFO_last_spec_ctrl(%rsp)
 1:          /* No Spectre v1 concerns.  Execution will hit VMRUN imminently. */
         .endm
-        ALTERNATIVE "", svm_vmentry_spec_ctrl, X86_FEATURE_SC_MSR_HVM
+        ALTERNATIVE_2 "", STR(call vmentry_virt_spec_ctrl), \
+                          X86_FEATURE_VIRT_SC_MSR_HVM, \
+                      svm_vmentry_spec_ctrl, X86_FEATURE_SC_MSR_HVM
 
         pop  %r15
         pop  %r14
@@ -111,7 +113,9 @@ __UNLIKELY_END(nsvm_hap)
             wrmsr
             mov    %al, CPUINFO_last_spec_ctrl(%rsp)
         .endm
-        ALTERNATIVE "", svm_vmexit_spec_ctrl, X86_FEATURE_SC_MSR_HVM
+        ALTERNATIVE_2 "", STR(call vmexit_virt_spec_ctrl), \
+                          X86_FEATURE_VIRT_SC_MSR_HVM, \
+                      svm_vmexit_spec_ctrl, X86_FEATURE_SC_MSR_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         stgi
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index c4ce3f75ab..56c7b30b32 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -610,6 +610,14 @@ static void svm_cpuid_policy_changed(struct vcpu *v)
     svm_intercept_msr(v, MSR_SPEC_CTRL,
                       cp->extd.ibrs ? MSR_INTERCEPT_NONE : MSR_INTERCEPT_RW);
 
+    /*
+     * Give access to MSR_VIRT_SPEC_CTRL if the guest has been told about it
+     * and the hardware implements it.
+     */
+    svm_intercept_msr(v, MSR_VIRT_SPEC_CTRL,
+                      cp->extd.virt_ssbd && cpu_has_virt_ssbd ?
+                      MSR_INTERCEPT_NONE : 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);
@@ -3099,6 +3107,53 @@ void svm_vmexit_handler(struct cpu_user_regs *regs)
     vmcb_set_vintr(vmcb, intr);
 }
 
+/* Called with GIF=0. */
+void vmexit_virt_spec_ctrl(void)
+{
+    struct cpu_info *info = get_cpu_info();
+    unsigned int val = info->xen_spec_ctrl;
+
+    /*
+     * On AMD we will never use MSR_SPEC_CTRL together with MSR_VIRT_SPEC_CTRL
+     * or any legacy way of setting SSBD, so reuse the spec_ctrl fields in
+     * cpu_info for context switching the other means of setting SSBD.
+     */
+    ASSERT(!boot_cpu_has(X86_FEATURE_SC_MSR_HVM));
+    if ( cpu_has_virt_ssbd )
+    {
+        unsigned int lo, hi;
+        struct vcpu *curr = current;
+
+        /*
+         * Need to read from the hardware because VIRT_SPEC_CTRL is not context
+         * switched by the hardware, and we allow the guest untrapped access to
+         * the register.
+         */
+        rdmsr(MSR_VIRT_SPEC_CTRL, lo, hi);
+        if ( val != lo )
+            wrmsr(MSR_VIRT_SPEC_CTRL, val, 0);
+        curr->arch.msrs->spec_ctrl.raw = lo;
+        info->last_spec_ctrl = val;
+    }
+}
+
+/* Called with GIF=0. */
+void vmentry_virt_spec_ctrl(void)
+{
+    struct cpu_info *info = get_cpu_info();
+    const struct vcpu *curr = current;
+    unsigned int val = curr->arch.msrs->spec_ctrl.raw;
+
+    ASSERT(!boot_cpu_has(X86_FEATURE_SC_MSR_HVM));
+    if ( val != info->last_spec_ctrl )
+    {
+        wrmsr(MSR_VIRT_SPEC_CTRL, val, 0);
+        info->last_spec_ctrl = val;
+    }
+
+    /* No Spectre v1 concerns.  Execution is going to hit VMRUN imminently. */
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/arch/x86/include/asm/cpufeatures.h b/xen/arch/x86/include/asm/cpufeatures.h
index b10154fc44..a2c37bfdd4 100644
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/cpufeatures.h
@@ -39,6 +39,7 @@ XEN_CPUFEATURE(SC_VERW_PV,        X86_SYNTH(23)) /* VERW used by Xen for PV */
 XEN_CPUFEATURE(SC_VERW_HVM,       X86_SYNTH(24)) /* VERW used by Xen for HVM */
 XEN_CPUFEATURE(SC_VERW_IDLE,      X86_SYNTH(25)) /* VERW used by Xen for idle */
 XEN_CPUFEATURE(XEN_SHSTK,         X86_SYNTH(26)) /* Xen uses CET Shadow Stacks */
+XEN_CPUFEATURE(VIRT_SC_MSR_HVM,   X86_SYNTH(27)) /* MSR_VIRT_SPEC_CTRL exposed to HVM */
 
 /* Bug words follow the synthetic words. */
 #define X86_NR_BUG 1
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 64b154b2d3..2c46e1485f 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -399,9 +399,12 @@ 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_MD_CLEAR)   ||
+            boot_cpu_has(X86_FEATURE_VIRT_SC_MSR_HVM) ||
             opt_eager_fpu)                           ? ""               : " 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"     : "",
            boot_cpu_has(X86_FEATURE_MD_CLEAR)        ? " MD_CLEAR"      : "");
@@ -1053,6 +1056,9 @@ void __init init_speculation_mitigations(void)
             setup_force_cpu_cap(X86_FEATURE_SC_MSR_HVM);
     }
 
+    if ( opt_msr_sc_hvm && cpu_has_virt_ssbd )
+        setup_force_cpu_cap(X86_FEATURE_VIRT_SC_MSR_HVM);
+
     /* If we have IBRS available, see whether we should use it. */
     if ( has_spec_ctrl && ibrs )
         default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
-- 
2.34.1