linux-kernel - Re: [RFC][PATCH 2/3] x86/uaccess: Implement unsafe_try_cmpxchg

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Date:   Mon, 20 Dec 2021 17:30:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tglx@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-api@...r.kernel.org, x86@...nel.org,
        pjt@...gle.com, posk@...gle.com, avagin@...gle.com,
        jannh@...gle.com, tdelisle@...terloo.ca, posk@...k.io
Subject: Re: [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@...radead.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.
> 
>