On Wed, Jul 13, 2022, Vitaly Kuznetsov wrote:
> It seems to be a misconception that "A" places an u64 operand to
> EAX:EDX, at least with GCC11.
It's not a misconception, it's just that the "A" trick only works for 32-bit
binaries. For 64-bit, the 64-bit integer fits into "rax" without needing to spill
into "rdx".
I swear I had fixed this, but apparently I had only done that locally and never
pushed/posted the changes :-/
> 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
>
