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

From: Vishal Annapurve
Date: Wed Sep 14 2022 - 20:05:26 EST


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