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Subject[PATCH v2] KVM: selftests: Use "a" and "d" to set EAX/EDX for wrmsr_safe()
From
From: Vitaly Kuznetsov <vkuznets@redhat.com>

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: 3b23054cd3f5 ("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>
---
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..71e942ffac77 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"(val & -1u), "d"(val >> 32), "c"(msr));
}

uint64_t vm_get_page_table_entry(struct kvm_vm *vm, struct kvm_vcpu *vcpu,
base-commit: b624ae35418ce9424f639f8ffa2568e7674c262b
--
2.37.0.144.g8ac04bfd2-goog

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