Re: [PATCH RFC] vhost: set dirty log bits without pinning pages on x86

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 13, 2026 at 09:29:37AM +0800, zhaoguohan@kylinos.cn wrote:
> On Thu, Nov 12, 2026 at 05:53:47AM -0500, Michael S. Tsirkin wrote:
> > vhost had a TODO to replace page pinning with a version
> > of set_bit that works on userspace memory, since forever.
> >
> > It is really best to avoid GUP on fast path entirely.
> > Implement set_user_bit and use it in vhost, to do just that.
> [...]
> > lightly tested. pls test and resubmit with your tool.
> 
> Thanks, this is much simpler than my version. Two comments on
> set_bit_user() before I resubmit:
> 
> > +	int __err = 0;							\
> > +	__uaccess_begin();						\
> > +	asm volatile("\n"						\
> > +		     "1: " LOCK_PREFIX "orb %[mask], %[ptr]\n"		\
> > +		     "2:\n"						\
> > +		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
> > +					   %[errout])			\
> > +		     : [errout] "=r" (__err),				\
> > +		       [ptr] "+m" (*(u8 __user *)(addr))		\
> 
> errout is only written on the fixup path, so it needs "+r", as in
> __try_cmpxchg_user_asm(). With "=r" gcc drops the initialisation; here
> (gcc 13.3, x86_64_defconfig) it even shares a register with mask:
> 
>      1a3:	lock or %dl,(%rax)
>      1a9:	test   %edx,%edx
>      1ab:	jne    1bd <log_write+0x5d>
>      ...
>      1bd:	mov    $0xfffffff2,%eax
> 
> so every call returns -EFAULT, and log_write() stops after the first
> page.
> 
> > +	__uaccess_begin();						\
> 
> No access_ok() here. vhost checks the log range when it is set up

exactly

> and
> I couldn't find a way around that, but GUP used to reject kernel
> addresses by itself. user_access_begin() would keep the check (and the
> nospec barrier) local.

What we *definitely* don't want is nospec on fast path.

> Something like this, build-tested on x86_64 and
> i386 only:
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -597,19 +597,23 @@ _label:									\
>  /* set_bit() on a byte of user memory; returns -EFAULT on fault. */
>  #define set_bit_user(nr, addr)					\
>  ({									\
> -	int __err = 0;							\
> -	__uaccess_begin();						\
> -	asm volatile("\n"						\
> -		     "1: " LOCK_PREFIX "orb %[mask], %[ptr]\n"		\
> -		     "2:\n"						\
> -		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
> -					   %[errout])			\
> -		     : [errout] "=r" (__err),				\
> -		       [ptr] "+m" (*(u8 __user *)(addr))		\
> -		     : [mask] "q" ((unsigned char)BIT(nr))		\
> -		     : "memory", "cc");				\
> -	__uaccess_end();						\
> -	__err ? -EFAULT : 0;						\
> +	u8 __user *__p = (addr);					\
> +	int __err = -EFAULT;						\
> +	if (user_access_begin(__p, sizeof(*__p))) {			\
> +		__err = 0;						\
> +		asm volatile("\n"					\
> +			     "1: " LOCK_PREFIX "orb %[mask], %[ptr]\n"	\
> +			     "2:\n"					\
> +			     _ASM_EXTABLE_TYPE_REG(1b, 2b,		\
> +						   EX_TYPE_EFAULT_REG,	\
> +						   %[errout])		\
> +			     : [errout] "+r" (__err),			\
> +			       [ptr] "+m" (*__p)			\
> +			     : [mask] "q" ((unsigned char)BIT(nr))	\
> +			     : "memory", "cc");			\
> +		user_access_end();					\
> +	}								\
> +	__err;								\
>  })
>  
>  /*
> 
> If this looks right, I'll fold it in, run my test in a VM and resubmit
> with the test as a second patch.
> 
> Thanks,
> Khan


Given the point is to optimize this code, let's optimize.
vhost pre-validates addresses so accesses are safe without
re-checking later.


-- 
MST