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: 3b23054cd3 ("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
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