Re: [PATCH RFC] selftests: kvm: fix sync_regs_test with newer gccs

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/10/19 16:57, Vitaly Kuznetsov wrote:
> 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.
> 
> Disassembly show the following:
> 
> 00000000004019e0 <guest_code>:
>   4019e0:       55                      push   %rbp
>   4019e1:       89 dd                   mov    %ebx,%ebp
>   4019e3:       53                      push   %rbx
>   4019e4:       48 83 ec 08             sub    $0x8,%rsp
>   4019e8:       0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
>   4019ef:       00
>   4019f0:       31 c9                   xor    %ecx,%ecx
>   4019f2:       ba 10 90 40 00          mov    $0x409010,%edx
>   4019f7:       be 02 00 00 00          mov    $0x2,%esi
>   4019fc:       31 c0                   xor    %eax,%eax
>   4019fe:       bf 01 00 00 00          mov    $0x1,%edi
>   401a03:       83 c5 01                add    $0x1,%ebp
>   401a06:       e8 15 2b 00 00          callq  404520 <ucall>
>   401a0b:       89 eb                   mov    %ebp,%ebx
>   401a0d:       eb e1                   jmp    4019f0 <guest_code+0x10>
>   401a0f:       90                      nop
> 
> and apparently this is broken. If we add 'volatile' qualifier to 'stage'
> we get the following code:
> 
> 00000000004019e0 <guest_code>:
>   4019e0:       53                      push   %rbx
>   4019e1:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
>   4019e8:       31 c9                   xor    %ecx,%ecx
>   4019ea:       ba 10 90 40 00          mov    $0x409010,%edx
>   4019ef:       be 02 00 00 00          mov    $0x2,%esi
>   4019f4:       31 c0                   xor    %eax,%eax
>   4019f6:       bf 01 00 00 00          mov    $0x1,%edi
>   4019fb:       83 c3 01                add    $0x1,%ebx
>   4019fe:       e8 1d 2b 00 00          callq  404520 <ucall>
>   401a03:       eb e3                   jmp    4019e8 <guest_code+0x8>
>   401a05:       66 66 2e 0f 1f 84 00    data16 nopw %cs:0x0(%rax,%rax,1)
>   401a0c:       00 00 00 00
> 
> and everything seems to work. The only problem is that I now get a new
> warning from gcc:
> 
> x86_64/sync_regs_test.c: In function ‘guest_code’:
> x86_64/sync_regs_test.c:25:6: warning: optimization may eliminate reads
>  and/or writes to register variables [-Wvolatile-register-var]
> 
> checkpatch.pl doesn't like me either:
> 
> "WARNING: Use of volatile is usually wrong: see
>  Documentation/process/volatile-considered-harmful.rst"
> 
> I can think of an 'ultimate' solution to open code ucall() in a single
> asm block making sure the register we need is preserved but this looks
> like an overkill.

It is probably the best idea though.  It's a processor-specific test
anyway.  It also doesn't need the full ucall(), just a single in
instruction will do.

Paolo

> Fixes: 204c91eff798a ("KVM: selftests: do not blindly clobber registers in guest asm")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  tools/testing/selftests/kvm/x86_64/sync_regs_test.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 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 11c2a70a7b87..25c54250d591 100644
> --- a/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/sync_regs_test.c
> @@ -28,7 +28,7 @@ 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");
> +	register volatile u32 stage asm("rbx");
>  	for (;;) {
>  		GUEST_SYNC(0);
>  		stage++;
>