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

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Message-ID: <YcG32Ytj0zUAW/B2@hirez.programming.kicks-ass.net>
Date:   Tue, 21 Dec 2021 12:17:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
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 Mon, Dec 20, 2021 at 05:30:05PM +0000, Sean Christopherson wrote:
> 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.

Sure, no problem. I currently still need to audit parts of mm/ and
do the smp-wake-idle bits before I repost -- that and take a xmas break
ofcourse :-) So it'll be a while before I repost this.