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selftests: kvm: fix sync_regs_test with newer gccs

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Commit 204c91eff7 ("KVM: selftests: do not blindly clobber registers in
 guest asm") was intended to make test more gcc-proof, however, the result
is exactly the opposite: on newer gccs (e.g. 8.2.1) the test breaks with

==== Test Assertion Failure ====
  x86_64/sync_regs_test.c:168: run->s.regs.regs.rbx == 0xBAD1DEA + 1
  pid=14170 tid=14170 - Invalid argument
     1	0x00000000004015b3: main at sync_regs_test.c:166 (discriminator 6)
     2	0x00007f413fb66412: ?? ??:0
     3	0x000000000040191d: _start at ??:?
  rbx sync regs value incorrect 0x1.

Apparently, compile is still free to play games with registers even
when they have variables attached.

Re-write guest code with 'asm volatile' by embedding ucall there and
making sure rbx is preserved.

Fixes: 204c91eff7 ("KVM: selftests: do not blindly clobber registers in guest asm")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2019-10-08 20:08:08 +02:00 committed by Paolo Bonzini
parent 11eada4718
commit ef40598098
1 changed files with 11 additions and 10 deletions
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21
tools/testing/selftests/kvm/x86_64/sync_regs_test.c
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@ -22,18 +22,19 @@
#define VCPU_ID 5
#define UCALL_PIO_PORT ((uint16_t)0x1000)
/*
* ucall is embedded here to protect against compiler reshuffling registers
* before calling a function. In this test we only need to get KVM_EXIT_IO
* vmexit and preserve RBX, no additional information is needed.
*/
void guest_code(void)
{
/*
* use a callee-save register, otherwise the compiler
* saves it around the call to GUEST_SYNC.
*/
register u32 stage asm("rbx");
for (;;) {
GUEST_SYNC(0);
stage++;
asm volatile ("" : : "r" (stage));
}
asm volatile("1: in %[port], %%al\n"
"add $0x1, %%rbx\n"
"jmp 1b"
: : [port] "d" (UCALL_PIO_PORT) : "rax", "rbx");
}
static void compare_regs(struct kvm_regs *left, struct kvm_regs *right)