KVM: selftests: Use "a" and "d" to set EAX/EDX for wrmsr_safe()

KVM: selftests: Use "a" and "d" to set EAX/EDX for wrmsr_safe()

Do not use GCC's "A" constraint to load EAX:EDX in wrmsr_safe().  Per
GCC's documenation on x86-specific constraints, "A" will not actually
load a 64-bit value into EAX:EDX on x86-64.

  The a and d registers. This class is used for instructions that return
  double word results in the ax:dx register pair. Single word values will
  be allocated either in ax or dx. For example on i386 the following
  implements rdtsc:

  unsigned long long rdtsc (void)
  {
    unsigned long long tick;
    __asm__ __volatile__("rdtsc":"=A"(tick));
    return tick;
  }

  This is not correct on x86-64 as it would allocate tick in either ax or
  dx. You have to use the following variant instead:

  unsigned long long rdtsc (void)
  {
    unsigned int tickl, tickh;
    __asm__ __volatile__("rdtsc":"=a"(tickl),"=d"(tickh));
    return ((unsigned long long)tickh << 32)|tickl;
  }

Because a u64 fits in a single 64-bit register, using "A" for selftests,
which are 64-bit only, results in GCC loading the value into either RAX
or RDX instead of splitting it across EAX:EDX.

E.g.:

  kvm_exit:             reason MSR_WRITE rip 0x402919 info 0 0
  kvm_msr:              msr_write 40000118 = 0x60000000001 (#GP)
...

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 53 08             mov    0x8(%rbx),%rdx
  89 d0                   mov    %edx,%eax
  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

Fixes: 17919ed2d5a3 ("KVM: selftests: Add x86-64 support for exception fixup")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
[sean: use "& -1u", provide GCC blurb and link to documentation]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220714011115.3135828-1-seanjc@google.com