x86/spec-ctrl: Rework spec_ctrl_flags context switching

We are shortly going to need to context switch new bits in both the vcpu and
S3 paths.  Introduce SCF_IST_MASK and SCF_DOM_MASK, and rework d->arch.verw
into d->arch.spec_ctrl_flags to accommodate.

No functional change.

This is part of XSA-407.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 5796912f72)

xen/arch/x86/acpi/power.c

@@ -246,8 +246,8 @@ static int enter_state(u32 state)
         error = 0;
 
     ci = get_cpu_info();
-    /* Avoid NMI/#MC using MSR_SPEC_CTRL until we've reloaded microcode. */
-    ci->spec_ctrl_flags &= ~SCF_ist_wrmsr;
+    /* Avoid NMI/#MC using unsafe MSRs until we've reloaded microcode. */
+    ci->spec_ctrl_flags &= ~SCF_IST_MASK;
 
     ACPI_FLUSH_CPU_CACHE();
 
@@ -290,8 +290,8 @@ static int enter_state(u32 state)
     if ( !recheck_cpu_features(0) )
         panic("Missing previously available feature(s)\n");
 
-    /* Re-enabled default NMI/#MC use of MSR_SPEC_CTRL. */
-    ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
+    /* Re-enabled default NMI/#MC use of MSRs now microcode is loaded. */
+    ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_IST_MASK);
 
     if ( boot_cpu_has(X86_FEATURE_IBRSB) || boot_cpu_has(X86_FEATURE_IBRS) )
     {

xen/arch/x86/domain.c

@@ -1838,10 +1838,10 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
             }
         }
 
-        /* Update the top-of-stack block with the VERW disposition. */
-        info->spec_ctrl_flags &= ~SCF_verw;
-        if ( nextd->arch.verw )
-            info->spec_ctrl_flags |= SCF_verw;
+        /* Update the top-of-stack block with the new spec_ctrl settings. */
+        info->spec_ctrl_flags =
+            (info->spec_ctrl_flags       & ~SCF_DOM_MASK) |
+            (nextd->arch.spec_ctrl_flags &  SCF_DOM_MASK);
     }
 
     sched_context_switched(prev, next);

xen/arch/x86/spec_ctrl.c

@@ -981,9 +981,12 @@ void spec_ctrl_init_domain(struct domain *d)
 {
     bool pv = is_pv_domain(d);
 
-    d->arch.verw =
-        (pv ? opt_md_clear_pv : opt_md_clear_hvm) ||
-        (opt_fb_clear_mmio && is_iommu_enabled(d));
+    bool verw = ((pv ? opt_md_clear_pv : opt_md_clear_hvm) ||
+                 (opt_fb_clear_mmio && is_iommu_enabled(d)));
+
+    d->arch.spec_ctrl_flags =
+        (verw   ? SCF_verw         : 0) |
+        0;
 }
 
 void __init init_speculation_mitigations(void)

xen/include/asm-x86/domain.h

@@ -308,8 +308,7 @@ struct arch_domain
     uint32_t pci_cf8;
     uint8_t cmos_idx;
 
-    /* Use VERW on return-to-guest for its flushing side effect. */
-    bool verw;
+    uint8_t spec_ctrl_flags; /* See SCF_DOM_MASK */
 
     union {
         struct pv_domain pv;

xen/include/asm-x86/spec_ctrl.h

@@ -20,12 +20,40 @@
 #ifndef __X86_SPEC_CTRL_H__
 #define __X86_SPEC_CTRL_H__
 
-/* Encoding of cpuinfo.spec_ctrl_flags */
+/*
+ * Encoding of:
+ *   cpuinfo.spec_ctrl_flags
+ *   default_spec_ctrl_flags
+ *   domain.spec_ctrl_flags
+ *
+ * Live settings are in the top-of-stack block, because they need to be
+ * accessable when XPTI is active.  Some settings are fixed from boot, some
+ * context switched per domain, and some inhibited in the S3 path.
+ */
 #define SCF_use_shadow (1 << 0)
 #define SCF_ist_wrmsr  (1 << 1)
 #define SCF_ist_rsb    (1 << 2)
 #define SCF_verw       (1 << 3)
 
+/*
+ * The IST paths (NMI/#MC) can interrupt any arbitrary context.  Some
+ * functionality requires updated microcode to work.
+ *
+ * On boot, this is easy; we load microcode before figuring out which
+ * speculative protections to apply.  However, on the S3 resume path, we must
+ * be able to disable the configured mitigations until microcode is reloaded.
+ *
+ * These are the controls to inhibit on the S3 resume path until microcode has
+ * been reloaded.
+ */
+#define SCF_IST_MASK (SCF_ist_wrmsr)
+
+/*
+ * Some speculative protections are per-domain.  These settings are merged
+ * into the top-of-stack block in the context switch path.
+ */
+#define SCF_DOM_MASK (SCF_verw)
+
 #ifndef __ASSEMBLY__
 
 #include <asm/alternative.h>

xen/include/asm-x86/spec_ctrl_asm.h

@@ -248,9 +248,6 @@
 
 /*
  * Use in IST interrupt/exception context.  May interrupt Xen or PV context.
- * Fine grain control of SCF_ist_wrmsr is needed for safety in the S3 resume
- * path to avoid using MSR_SPEC_CTRL before the microcode introducing it has
- * been reloaded.
  */
 .macro SPEC_CTRL_ENTRY_FROM_INTR_IST
 /*