Re: [PATCH 2/3] KVM: selftests: Fix wrmsr_safe()

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 13, 2022, Sean Christopherson wrote:
> 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 :-/

Ugh, I have a feeling I fixed RDMSR and then forgot about WRSMR.

> > 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

Huh.  This is wrong.  RAX is loaded with the full 64-bit value.  It doesn't matter
for WRMSR because WRMSR only consumes EAX, but it's wrong.  I can't for the life
of me figure out why casting to a u32 doesn't force the compiler to truncate the
value.  Truncation in other places most definitely works, and the compiler loads
only EAX and EDX when using a hardcoded value, e.g. -1ull, so the input isn't
messed up.  There's no 32-bit loads of EAX, so no implicit truncation of RAX[63:32].

gcc-{7,9,11} and clang-13 generate the same code, so either it's a really
longstanding bug, or maybe some funky undocumented behavior?

If I use

	return kvm_asm_safe("wrmsr", "a"(val & -1u), "d"(val >> 32), "c"(msr));

then the result is as expected:

  48 8b 53 08             mov    0x8(%rbx),%rdx
  89 d0                   mov    %edx,%eax
  48 c1 ea 20             shr    $0x20,%rdx

I'll post a v2 of just this patch on your behalf, I also reworded the changelog
to include the gcc documentation that talks about the behavior of "A".