* [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()
@ 2022-06-07 15:00 Jan Beulich
2022-06-07 15:16 ` Paolo Bonzini
0 siblings, 1 reply; 2+ messages in thread
From: Jan Beulich @ 2022-06-07 15:00 UTC (permalink / raw)
To: Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
Cc: lkml, the arch/x86 maintainers, H. Peter Anvin, Peter Zijlstra,
Sean Christopherson, Paolo Bonzini
As noted (and fixed) a couple of times in the past, "=@cc<cond>" outputs
and clobbering of "cc" don't work well together. The compiler appears to
mean to reject such, but doesn't - in its upstream form - quite manage
to yet for "cc". Furthermore two similar macros don't clobber "cc", and
clobbering "cc" is pointless in asm()-s for x86 anyway - the compiler
always assumes status flags to be clobbered there.
Fixes: 989b5db215a2 ("x86/uaccess: Implement macros for CMPXCHG on user addresses")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
The change I continue to carry locally actually allows specifying that
"cc" is _not_ clobbered; it only happens to improve detection of bad
situations like the one here.
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -439,7 +439,7 @@ do { \
[ptr] "+m" (*_ptr), \
[old] "+a" (__old) \
: [new] ltype (__new) \
- : "memory", "cc"); \
+ : "memory"); \
if (unlikely(__err)) \
goto label; \
if (unlikely(!success)) \
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm()
2022-06-07 15:00 [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm() Jan Beulich
@ 2022-06-07 15:16 ` Paolo Bonzini
0 siblings, 0 replies; 2+ messages in thread
From: Paolo Bonzini @ 2022-06-07 15:16 UTC (permalink / raw)
To: Jan Beulich, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
Cc: lkml, the arch/x86 maintainers, H. Peter Anvin, Peter Zijlstra,
Sean Christopherson
On 6/7/22 17:00, Jan Beulich wrote:
> As noted (and fixed) a couple of times in the past, "=@cc<cond>" outputs
> and clobbering of "cc" don't work well together. The compiler appears to
> mean to reject such, but doesn't - in its upstream form - quite manage
> to yet for "cc". Furthermore two similar macros don't clobber "cc", and
> clobbering "cc" is pointless in asm()-s for x86 anyway - the compiler
> always assumes status flags to be clobbered there.
>
> Fixes: 989b5db215a2 ("x86/uaccess: Implement macros for CMPXCHG on user addresses")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> The change I continue to carry locally actually allows specifying that
> "cc" is _not_ clobbered; it only happens to improve detection of bad
> situations like the one here.
>
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -439,7 +439,7 @@ do { \
> [ptr] "+m" (*_ptr), \
> [old] "+a" (__old) \
> : [new] ltype (__new) \
> - : "memory", "cc"); \
> + : "memory"); \
> if (unlikely(__err)) \
> goto label; \
> if (unlikely(!success)) \
>
Queued, thanks.
Paolo
^ permalink raw reply [flat|nested] 2+ messages in thread
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2022-06-07 15:00 [PATCH] x86: drop bogus "cc" clobber from __try_cmpxchg_user_asm() Jan Beulich
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