From: Vitaly Kuznetsov vkuznets@redhat.com
[ Upstream commit ef4059809890f732c69cc1726d3a9a108a832a2f ]
Commit 204c91eff798a ("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: 204c91eff798a ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- .../selftests/kvm/x86_64/sync_regs_test.c | 21 ++++++++++--------- 1 file changed, 11 insertions(+), 10 deletions(-)
diff --git a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c index 11c2a70a7b87a..5c82242562943 100644 --- a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c +++ b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c @@ -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)