[V2 PATCH 4/8] KVM: selftests: x86: Precompute the result for is_{intel,amd}_cpu()

[Vishal Annapurve <vannapurve@xxxxxxxxxx>]

Cache the vendor CPU type in a global variable so that multiple calls
to is_intel_cpu() do not need to re-execute CPUID.

Add cpu vendor check in kvm_hypercall() so that it executes correct
vmcall/vmmcall instruction when running on Intel/AMD hosts. This avoids
exit to KVM which anyway tries to patch the instruction according to
the cpu type.

As part of this change, sync the global variable is_cpu_amd into the
guest so the guest can determine which hypercall instruction to use
without needing to re-execute CPUID for every hypercall.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Vishal Annapurve <vannapurve@xxxxxxxxxx>
---
 .../testing/selftests/kvm/lib/x86_64/processor.c  | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index 25ae972f5c71..c0ae938772f6 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -19,6 +19,7 @@
 #define MAX_NR_CPUID_ENTRIES 100
 
 vm_vaddr_t exception_handlers;
+static bool is_cpu_amd;
 
 static void regs_dump(FILE *stream, struct kvm_regs *regs, uint8_t indent)
 {
@@ -1019,7 +1020,7 @@ static bool cpu_vendor_string_is(const char *vendor)
 
 bool is_intel_cpu(void)
 {
-	return cpu_vendor_string_is("GenuineIntel");
+	return (is_cpu_amd == false);
 }
 
 /*
@@ -1027,7 +1028,7 @@ bool is_intel_cpu(void)
  */
 bool is_amd_cpu(void)
 {
-	return cpu_vendor_string_is("AuthenticAMD");
+	return (is_cpu_amd == true);
 }
 
 void kvm_get_cpu_address_width(unsigned int *pa_bits, unsigned int *va_bits)
@@ -1182,9 +1183,15 @@ uint64_t kvm_hypercall(uint64_t nr, uint64_t a0, uint64_t a1, uint64_t a2,
 {
 	uint64_t r;
 
-	asm volatile("vmcall"
+	if (is_amd_cpu())
+		asm volatile("vmmcall"
 		     : "=a"(r)
 		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3));
+	else
+		asm volatile("vmcall"
+		     : "=a"(r)
+		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3));
+
 	return r;
 }
 
@@ -1314,8 +1321,10 @@ bool vm_is_unrestricted_guest(struct kvm_vm *vm)
 
 void kvm_selftest_arch_init(void)
 {
+	is_cpu_amd = cpu_vendor_string_is("AuthenticAMD");
 }
 
 void kvm_arch_post_vm_elf_load(struct kvm_vm *vm)
 {
+	sync_global_to_guest(vm, is_cpu_amd);
 }
-- 
2.37.2.789.g6183377224-goog