It seems to be a misconception that "A" places an u64 operand to
EAX:EDX, at least with GCC11.
While writing a new test, I've noticed that wrmsr_safe() tries putting
garbage to the upper bits of the MSR, e.g.:
kvm_exit: reason MSR_WRITE rip 0x402919 info 0 0
kvm_msr: msr_write 40000118 = 0x60000000001 (#GP)
...
when it was supposed to write '1'. Apparently, "A" works the same as
"a" and not as EAX/EDX. Here's the relevant disassembled part:
With "A":
48 8b 43 08 mov 0x8(%rbx),%rax
49 b9 ba da ca ba 0a movabs $0xabacadaba,%r9
00 00 00
4c 8d 15 07 00 00 00 lea 0x7(%rip),%r10 # 402f44 <guest_msr+0x34>
4c 8d 1d 06 00 00 00 lea 0x6(%rip),%r11 # 402f4a <guest_msr+0x3a>
0f 30 wrmsr
With "a"/"d":
48 8b 43 08 mov 0x8(%rbx),%rax
48 89 c2 mov %rax,%rdx
48 c1 ea 20 shr $0x20,%rdx
49 b9 ba da ca ba 0a movabs $0xabacadaba,%r9
00 00 00
4c 8d 15 07 00 00 00 lea 0x7(%rip),%r10 # 402fc3 <guest_msr+0xb3>
4c 8d 1d 06 00 00 00 lea 0x6(%rip),%r11 # 402fc9 <guest_msr+0xb9>
0f 30 wrmsr
I was only able to find one online reference that "A" gives "eax and
edx combined into a 64-bit integer", other places don't mention it at
all.
Fixes: 3b23054cd3f5 ("KVM: selftests: Add x86-64 support for exception fixup")
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
tools/testing/selftests/kvm/include/x86_64/processor.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
index 79dcf6be1b47..3d412c578e78 100644
--- a/tools/testing/selftests/kvm/include/x86_64/processor.h
+++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
@@ -612,7 +612,7 @@ static inline uint8_t rdmsr_safe(uint32_t msr, uint64_t *val)
static inline uint8_t wrmsr_safe(uint32_t msr, uint64_t val)
{
- return kvm_asm_safe("wrmsr", "A"(val), "c"(msr));
+ return kvm_asm_safe("wrmsr", "a"((u32)val), "d"(val >> 32), "c"(msr));
}
uint64_t vm_get_page_table_entry(struct kvm_vm *vm, struct kvm_vcpu *vcpu,
--
2.35.3
