x86/spec-ctrl: Drop SPEC_CTRL_{ENTRY_FROM,EXIT_TO}_HVM

These were written before Spectre/Meltdown went public, and there was large
uncertainty in how the protections would evolve.  As it turns out, they're
very specific to Intel hardware, and not very suitable for AMD.

Drop the macros, opencoding the relevant subset of functionality, and leaving
grep-fodder to locate the logic.  No change at all for VT-x.

For AMD, the only relevant piece of functionality is DO_OVERWRITE_RSB,
although we will soon be adding (different) logic to handle MSR_SPEC_CTRL.

This has a marginal improvement of removing an unconditional pile of long-nops
from the vmentry/exit path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 95b13fa43e0753b7514bef13abe28253e8614f62
master date: 2022-01-20 16:32:11 +0000

3 files changed, 13 insertions, 19 deletions

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index e208a4b32a..276215d36a 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -59,7 +59,7 @@ __UNLIKELY_END(nsvm_hap)
         mov VCPUMSR_spec_ctrl_raw(%rax), %eax
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        SPEC_CTRL_EXIT_TO_HVM   /* Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: cd */
+        /* SPEC_CTRL_EXIT_TO_SVM   (nothing currently) */
 
         pop  %r15
         pop  %r14
@@ -86,7 +86,8 @@ __UNLIKELY_END(nsvm_hap)
 
         GET_CURRENT(bx)
 
-        SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
+        /* SPEC_CTRL_ENTRY_FROM_SVM    Req: b=curr %rsp=regs/cpuinfo, Clob: ac  */
+        ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         stgi
diff --git a/xen/arch/x86/hvm/vmx/entry.S b/xen/arch/x86/hvm/vmx/entry.S
index 27c8c5ca49..30139ae58e 100644
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -33,7 +33,9 @@ ENTRY(vmx_asm_vmexit_handler)
         movb $1,VCPU_vmx_launched(%rbx)
         mov  %rax,VCPU_hvm_guest_cr2(%rbx)
 
-        SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
+        /* SPEC_CTRL_ENTRY_FROM_VMX    Req: b=curr %rsp=regs/cpuinfo, Clob: acd */
+        ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
+        ALTERNATIVE "", DO_SPEC_CTRL_ENTRY_FROM_HVM, X86_FEATURE_SC_MSR_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         /* Hardware clears MSR_DEBUGCTL on VMExit.  Reinstate it if debugging Xen. */
@@ -80,7 +82,9 @@ UNLIKELY_END(realmode)
         mov VCPUMSR_spec_ctrl_raw(%rax), %eax
 
         /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */
-        SPEC_CTRL_EXIT_TO_HVM   /* Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: cd */
+        /* SPEC_CTRL_EXIT_TO_VMX   Req: a=spec_ctrl %rsp=regs/cpuinfo, Clob: cd */
+        ALTERNATIVE "", DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM
+        ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)), X86_FEATURE_SC_VERW_HVM
 
         mov  VCPU_hvm_guest_cr2(%rbx),%rax
 
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h b/xen/include/asm-x86/spec_ctrl_asm.h
index cb34299a86..2b3f123cb5 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -68,14 +68,16 @@
  *
  * The following ASM fragments implement this algorithm.  See their local
  * comments for further details.
- *  - SPEC_CTRL_ENTRY_FROM_HVM
  *  - SPEC_CTRL_ENTRY_FROM_PV
  *  - SPEC_CTRL_ENTRY_FROM_INTR
  *  - SPEC_CTRL_ENTRY_FROM_INTR_IST
  *  - SPEC_CTRL_EXIT_TO_XEN_IST
  *  - SPEC_CTRL_EXIT_TO_XEN
  *  - SPEC_CTRL_EXIT_TO_PV
- *  - SPEC_CTRL_EXIT_TO_HVM
+ *
+ * Additionally, the following grep-fodder exists to find the HVM logic.
+ *  - SPEC_CTRL_ENTRY_FROM_{SVM,VMX}
+ *  - SPEC_CTRL_EXIT_TO_{SVM,VMX}
  */
 
 .macro DO_OVERWRITE_RSB tmp=rax
@@ -225,12 +227,6 @@
     wrmsr
 .endm
 
-/* Use after a VMEXIT from an HVM guest. */
-#define SPEC_CTRL_ENTRY_FROM_HVM                                        \
-    ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM;           \
-    ALTERNATIVE "", DO_SPEC_CTRL_ENTRY_FROM_HVM,                        \
-        X86_FEATURE_SC_MSR_HVM
-
 /* Use after an entry from PV context (syscall/sysenter/int80/int82/etc). */
 #define SPEC_CTRL_ENTRY_FROM_PV                                         \
     ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_PV;            \
@@ -255,13 +251,6 @@
     ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)),           \
         X86_FEATURE_SC_VERW_PV
 
-/* Use when exiting to HVM guest context. */
-#define SPEC_CTRL_EXIT_TO_HVM                                           \
-    ALTERNATIVE "",                                                     \
-        DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM;             \
-    ALTERNATIVE "", __stringify(verw CPUINFO_verw_sel(%rsp)),           \
-        X86_FEATURE_SC_VERW_HVM
-
 /*
  * 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