[Xen-devel] [PATCH -tip v4 2/4] x86: xen: kvm: Gather the definition of emulate prefixes

[Masami Hiramatsu <mhiramat@xxxxxxxxxx>]

Gather the emulate prefixes, which forcibly make the following
instruction emulated on virtualization, in one place.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 arch/x86/include/asm/emulate_prefix.h |   14 ++++++++++++++
 arch/x86/include/asm/xen/interface.h  |   11 ++++-------
 arch/x86/kvm/x86.c                    |    4 +++-
 3 files changed, 21 insertions(+), 8 deletions(-)
 create mode 100644 arch/x86/include/asm/emulate_prefix.h

diff --git a/arch/x86/include/asm/emulate_prefix.h 
b/arch/x86/include/asm/emulate_prefix.h
new file mode 100644
index 000000000000..70f5b98a5286
--- /dev/null
+++ b/arch/x86/include/asm/emulate_prefix.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_EMULATE_PREFIX_H
+#define _ASM_X86_EMULATE_PREFIX_H
+
+/*
+ * Virt escape sequences to trigger instruction emulation;
+ * ideally these would decode to 'whole' instruction and not destroy
+ * the instruction stream; sadly this is not true for the 'kvm' one :/
+ */
+
+#define __XEN_EMULATE_PREFIX  0x0f,0x0b,0x78,0x65,0x6e  /* ud2 ; .ascii "xen" 
*/
+#define __KVM_EMULATE_PREFIX  0x0f,0x0b,0x6b,0x76,0x6d /* ud2 ; .ascii "kvm" */
+
+#endif
diff --git a/arch/x86/include/asm/xen/interface.h 
b/arch/x86/include/asm/xen/interface.h
index 62ca03ef5c65..9139b3e86316 100644
--- a/arch/x86/include/asm/xen/interface.h
+++ b/arch/x86/include/asm/xen/interface.h
@@ -379,12 +379,9 @@ struct xen_pmu_arch {
  * Prefix forces emulation of some non-trapping instructions.
  * Currently only CPUID.
  */
-#ifdef __ASSEMBLY__
-#define XEN_EMULATE_PREFIX .byte 0x0f,0x0b,0x78,0x65,0x6e ;
-#define XEN_CPUID          XEN_EMULATE_PREFIX cpuid
-#else
-#define XEN_EMULATE_PREFIX ".byte 0x0f,0x0b,0x78,0x65,0x6e ; "
-#define XEN_CPUID          XEN_EMULATE_PREFIX "cpuid"
-#endif
+#include <asm/emulate_prefix.h>
+
+#define XEN_EMULATE_PREFIX __ASM_FORM(.byte __XEN_EMULATE_PREFIX ;)
+#define XEN_CPUID          XEN_EMULATE_PREFIX __ASM_FORM(cpuid)
 
 #endif /* _ASM_X86_XEN_INTERFACE_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 290c3c3efb87..5f8b0a60f48b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -68,6 +68,7 @@
 #include <asm/mshyperv.h>
 #include <asm/hypervisor.h>
 #include <asm/intel_pt.h>
+#include <asm/emulate_prefix.h>
 #include <clocksource/hyperv_timer.h>
 
 #define CREATE_TRACE_POINTS
@@ -5319,6 +5320,7 @@ EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
 
 int handle_ud(struct kvm_vcpu *vcpu)
 {
+       static const char kvm_emulate_prefix[] = { __KVM_EMULATE_PREFIX };
        int emul_type = EMULTYPE_TRAP_UD;
        enum emulation_result er;
        char sig[5]; /* ud2; .ascii "kvm" */
@@ -5327,7 +5329,7 @@ int handle_ud(struct kvm_vcpu *vcpu)
        if (force_emulation_prefix &&
            kvm_read_guest_virt(vcpu, kvm_get_linear_rip(vcpu),
                                sig, sizeof(sig), &e) == 0 &&
-           memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
+           memcmp(sig, kvm_emulate_prefix, sizeof(sig)) == 0) {
                kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
                emul_type = 0;
        }


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