[PATCH 1/3] amd/msr: implement VIRT_SPEC_CTRL for HVM guests on top of SPEC_CTRL

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Use the logic to set shadow SPEC_CTRL values in order to implement
support for VIRT_SPEC_CTRL (signaled by VIRT_SSBD CPUID flag) for HVM
guests. This includes using the spec_ctrl vCPU MSR variable to store
the guest set value of VIRT_SPEC_CTRL.SSBD.

Note that VIRT_SSBD is only set in the HVM max CPUID policy, as the
default should be to expose SPEC_CTRL only and support VIRT_SPEC_CTRL
for migration compatibility.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 docs/misc/xen-command-line.pandoc           |  5 +++--
 xen/arch/x86/cpuid.c                        |  7 +++++++
 xen/arch/x86/hvm/hvm.c                      |  1 +
 xen/arch/x86/include/asm/msr.h              |  6 +++++-
 xen/arch/x86/msr.c                          | 15 +++++++++++++++
 xen/arch/x86/spec_ctrl.c                    |  3 ++-
 xen/include/public/arch-x86/cpufeatureset.h |  2 +-
 7 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 6b3da6ddc1..081e10f80b 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2273,8 +2273,9 @@ to use.
 * `pv=` and `hvm=` offer control over all suboptions for PV and HVM guests
   respectively.
 * `msr-sc=` offers control over Xen's support for manipulating `MSR_SPEC_CTRL`
-  on entry and exit.  These blocks are necessary to virtualise support for
-  guests and if disabled, guests will be unable to use IBRS/STIBP/SSBD/etc.
+  and/or `MSR_VIRT_SPEC_CTRL` on entry and exit.  These blocks are necessary to
+  virtualise support for guests and if disabled, guests will be unable to use
+  IBRS/STIBP/SSBD/etc.
 * `rsb=` offers control over whether to overwrite the Return Stack Buffer /
   Return Address Stack on entry to Xen.
 * `md-clear=` offers control over whether to use VERW to flush
diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index e24dd283e7..29b4cfc9e6 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -543,6 +543,13 @@ static void __init calculate_hvm_max_policy(void)
         __clear_bit(X86_FEATURE_IBRSB, hvm_featureset);
         __clear_bit(X86_FEATURE_IBRS, hvm_featureset);
     }
+    else
+        /*
+         * If SPEC_CTRL is available VIRT_SPEC_CTRL can also be implemented as
+         * it's a subset of the controls exposed in SPEC_CTRL (SSBD only).
+         * Expose in the max policy for compatibility migration.
+         */
+        __set_bit(X86_FEATURE_VIRT_SSBD, hvm_featureset);
 
     /*
      * With VT-x, some features are only supported by Xen if dedicated
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index c4ddb8607d..3400c9299c 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1332,6 +1332,7 @@ static const uint32_t msrs_to_send[] = {
     MSR_INTEL_MISC_FEATURES_ENABLES,
     MSR_IA32_BNDCFGS,
     MSR_IA32_XSS,
+    MSR_VIRT_SPEC_CTRL,
     MSR_AMD64_DR0_ADDRESS_MASK,
     MSR_AMD64_DR1_ADDRESS_MASK,
     MSR_AMD64_DR2_ADDRESS_MASK,
diff --git a/xen/arch/x86/include/asm/msr.h b/xen/arch/x86/include/asm/msr.h
index ce4fe51afe..98f6b79e09 100644
--- a/xen/arch/x86/include/asm/msr.h
+++ b/xen/arch/x86/include/asm/msr.h
@@ -291,6 +291,7 @@ struct vcpu_msrs
 {
     /*
      * 0x00000048 - MSR_SPEC_CTRL
+     * 0xc001011f - MSR_VIRT_SPEC_CTRL
      *
      * For PV guests, this holds the guest kernel value.  It is accessed on
      * every entry/exit path.
@@ -301,7 +302,10 @@ struct vcpu_msrs
      * For SVM, the guest value lives in the VMCB, and hardware saves/restores
      * the host value automatically.  However, guests run with the OR of the
      * host and guest value, which allows Xen to set protections behind the
-     * guest's back.
+     * guest's back.  Use such functionality in order to implement support for
+     * VIRT_SPEC_CTRL as a shadow value of SPEC_CTRL and thus store the value
+     * of VIRT_SPEC_CTRL in this field, taking advantage of both MSRs having
+     * compatible layouts.
      *
      * We must clear/restore Xen's value before/after VMRUN to avoid unduly
      * influencing the guest.  In order to support "behind the guest's back"
diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index 4ac5b5a048..aa74cfde6c 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -381,6 +381,13 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t 
*val)
                ? K8_HWCR_TSC_FREQ_SEL : 0;
         break;
 
+    case MSR_VIRT_SPEC_CTRL:
+        if ( !cp->extd.virt_ssbd )
+            goto gp_fault;
+
+        *val = msrs->spec_ctrl.raw & SPEC_CTRL_SSBD;
+        break;
+
     case MSR_AMD64_DE_CFG:
         if ( !(cp->x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) )
             goto gp_fault;
@@ -666,6 +673,14 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
             wrmsr_tsc_aux(val);
         break;
 
+    case MSR_VIRT_SPEC_CTRL:
+        if ( !cp->extd.virt_ssbd )
+            goto gp_fault;
+
+        /* Only supports SSBD bit, the rest are ignored. */
+        msrs->spec_ctrl.raw = val & SPEC_CTRL_SSBD;
+        break;
+
     case MSR_AMD64_DE_CFG:
         /*
          * OpenBSD 6.7 will panic if writing to DE_CFG triggers a #GP:
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index ee862089b7..64b154b2d3 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -395,12 +395,13 @@ static void __init print_details(enum ind_thunk thunk, 
uint64_t caps)
      * mitigation support for guests.
      */
 #ifdef CONFIG_HVM
-    printk("  Support for HVM VMs:%s%s%s%s%s\n",
+    printk("  Support for HVM VMs:%s%s%s%s%s%s\n",
            (boot_cpu_has(X86_FEATURE_SC_MSR_HVM) ||
             boot_cpu_has(X86_FEATURE_SC_RSB_HVM) ||
             boot_cpu_has(X86_FEATURE_MD_CLEAR)   ||
             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_RSB_HVM)      ? " RSB"           : "",
            opt_eager_fpu                             ? " EAGER_FPU"     : "",
            boot_cpu_has(X86_FEATURE_MD_CLEAR)        ? " MD_CLEAR"      : "");
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 957df23b65..b9ab878ec1 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -265,7 +265,7 @@ XEN_CPUFEATURE(IBRS_SAME_MODE, 8*32+19) /*S  IBRS provides 
same-mode protection
 XEN_CPUFEATURE(NO_LMSL,       8*32+20) /*S  EFER.LMSLE no longer supported. */
 XEN_CPUFEATURE(AMD_PPIN,      8*32+23) /*   Protected Processor Inventory 
Number */
 XEN_CPUFEATURE(AMD_SSBD,      8*32+24) /*S  MSR_SPEC_CTRL.SSBD available */
-XEN_CPUFEATURE(VIRT_SSBD,     8*32+25) /*   MSR_VIRT_SPEC_CTRL.SSBD */
+XEN_CPUFEATURE(VIRT_SSBD,     8*32+25) /*!s MSR_VIRT_SPEC_CTRL.SSBD */
 XEN_CPUFEATURE(SSB_NO,        8*32+26) /*A  Hardware not vulnerable to SSB */
 XEN_CPUFEATURE(PSFD,          8*32+28) /*S  MSR_SPEC_CTRL.PSFD */
 
-- 
2.34.1