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SubjectRe: [RFC][PATCH 2/3] x86/uaccess: Implement unsafe_try_cmpxchg_user()
On Tue, Dec 14, 2021, Peter Zijlstra wrote:
> Do try_cmpxchg() loops on userspace addresses.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> @@ -501,6 +543,21 @@ do { \
> } while (0)
> #endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT
>
> +extern void __try_cmpxchg_user_wrong_size(void);
> +
> +#define unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({ \
> + __typeof__(*(_ptr)) __ret; \
> + switch (sizeof(__ret)) { \
> + case 4: __ret = __try_cmpxchg_user_asm("l", (_ptr), (_oldp), \
> + (_nval), _label); \
> + break; \
> + case 8: __ret = __try_cmpxchg_user_asm("q", (_ptr), (_oldp), \
> + (_nval), _label); \
> + break; \

Can we add support for 1-byte and 2-byte cmpxchg, and for using cmpxchg8b to handle
8-byte operations in 32-bit mode? Support for all the flavors (except 16-byte)
would allow KVM to use this in an emulator path that currently kmaps the target.
I'd be more than happy to help test the result.

Thanks!

> + default: __try_cmpxchg_user_wrong_size(); \
> + } \
> + __ret; })
> +
> /*
> * We want the unsafe accessors to always be inlined and use
> * the error labels - thus the macro games.
>
>

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