x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL

Hardware maintains both host and guest versions of MSR_SPEC_CTRL, but guests
run with the logical OR of both values.  Therefore, in principle we want to
clear Xen's value before entering the guest.  However, for migration
compatibility (future work), and for performance reasons with SEV-SNP guests,
we want the ability to use a nonzero value behind the guest's back.  Use
vcpu_msrs to hold this value, with the guest value in the VMCB.

On the VMEntry path, adjusting MSR_SPEC_CTRL must be done after CLGI so as to
be atomic with respect to NMIs/etc.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

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

@@ -55,11 +55,23 @@ __UNLIKELY_END(nsvm_hap)
         mov  %rsp, %rdi
         call svm_vmenter_helper
 
-        mov VCPU_arch_msrs(%rbx), %rax
-        mov VCPUMSR_spec_ctrl_raw(%rax), %eax
+        clgi
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        /* SPEC_CTRL_EXIT_TO_SVM   (nothing currently) */
+        /* SPEC_CTRL_EXIT_TO_SVM       Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
+        .macro svm_vmentry_spec_ctrl
+            mov    VCPU_arch_msrs(%rbx), %rax
+            movzbl CPUINFO_last_spec_ctrl(%rsp), %edx
+            mov    VCPUMSR_spec_ctrl_raw(%rax), %eax
+            cmp    %edx, %eax
+            je     1f /* Skip write if value is correct. */
+            mov    $MSR_SPEC_CTRL, %ecx
+            xor    %edx, %edx
+            wrmsr
+            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
 
         pop  %r15
         pop  %r14
@@ -78,7 +90,6 @@ __UNLIKELY_END(nsvm_hap)
         pop  %rsi
         pop  %rdi
 
-        clgi
         sti
         vmrun
 
@@ -86,8 +97,21 @@ __UNLIKELY_END(nsvm_hap)
 
         GET_CURRENT(bx)
 
-        /* SPEC_CTRL_ENTRY_FROM_SVM    Req: b=curr %rsp=regs/cpuinfo, Clob: ac  */
+        /* SPEC_CTRL_ENTRY_FROM_SVM    Req: %rsp=regs/cpuinfo         Clob: acd */
         ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
+
+        .macro svm_vmexit_spec_ctrl
+            /*
+             * Write to MSR_SPEC_CTRL unconditionally, for the RAS[:32]
+             * flushing side effect.
+             */
+            mov    $MSR_SPEC_CTRL, %ecx
+            movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
+            xor    %edx, %edx
+            wrmsr
+            mov    %al, CPUINFO_last_spec_ctrl(%rsp)
+        .endm
+        ALTERNATIVE "", svm_vmexit_spec_ctrl, X86_FEATURE_SC_MSR_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         stgi

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

@@ -297,6 +297,15 @@ struct vcpu_msrs
      *
      * For VT-x guests, the guest value is held in the MSR guest load/save
      * list.
+     *
+     * 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.
+     *
+     * 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"
+     * protections, we load this value (commonly 0) before VMRUN.
      */
     struct {
         uint32_t raw;

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

@@ -46,6 +46,9 @@
  *   - On VMX by using MSR load/save lists to have vmentry/exit atomically
  *     load/save the guest value.  Xen's value is loaded in regular code, and
  *     there is no need to use the shadow logic (below).
+ *   - On SVM by altering MSR_SPEC_CTRL inside the CLGI/STGI region.  This
+ *     makes the changes atomic with respect to NMIs/etc, so no need for
+ *     shadowing logic.
  *
  * Factor 2 is harder.  We maintain a shadow_spec_ctrl value, and a use_shadow
  * boolean in the per cpu spec_ctrl_flags.  The synchronous use is:

xen/arch/x86/x86_64/asm-offsets.c

@@ -126,6 +126,7 @@ void __dummy__(void)
     OFFSET(CPUINFO_pv_cr3, struct cpu_info, pv_cr3);
     OFFSET(CPUINFO_shadow_spec_ctrl, struct cpu_info, shadow_spec_ctrl);
     OFFSET(CPUINFO_xen_spec_ctrl, struct cpu_info, xen_spec_ctrl);
+    OFFSET(CPUINFO_last_spec_ctrl, struct cpu_info, last_spec_ctrl);
     OFFSET(CPUINFO_spec_ctrl_flags, struct cpu_info, spec_ctrl_flags);
     OFFSET(CPUINFO_root_pgt_changed, struct cpu_info, root_pgt_changed);
     OFFSET(CPUINFO_use_pv_cr3, struct cpu_info, use_pv_cr3);