Re: [RFC][PATCH 2/3] x86/uaccess: Implement unsafe_try_cmpxchg_user()

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
> > ---
> > @@ -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.