* Re: [PATCH RFC] vhost: set dirty log bits without pinning pages on x86
2026-11-13 1:29 ` zhaoguohan
@ 2026-11-13 5:47 ` Michael S. Tsirkin
2026-11-13 6:09 ` Michael S. Tsirkin
2026-11-13 13:51 ` Michael S. Tsirkin
2 siblings, 0 replies; 6+ messages in thread
From: Michael S. Tsirkin @ 2026-11-13 5:47 UTC (permalink / raw)
To: zhaoguohan
Cc: bp, dave.hansen, eperezma, hpa, jasowang, kvm, linux-kernel,
mingo, netdev, tglx, virtualization, x86
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
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH RFC] vhost: set dirty log bits without pinning pages on x86
2026-11-13 1:29 ` zhaoguohan
2026-11-13 5:47 ` Michael S. Tsirkin
@ 2026-11-13 6:09 ` Michael S. Tsirkin
2026-11-13 13:51 ` Michael S. Tsirkin
2 siblings, 0 replies; 6+ messages in thread
From: Michael S. Tsirkin @ 2026-11-13 6:09 UTC (permalink / raw)
To: zhaoguohan
Cc: bp, dave.hansen, eperezma, hpa, jasowang, kvm, linux-kernel,
mingo, netdev, tglx, virtualization, x86
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:
ah, subtle
>
> 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 and
> I couldn't find a way around that, but GUP used to reject kernel
> addresses by itself.
just a side effect of the api, unintentional.
> user_access_begin() would keep the check (and the
> nospec barrier) local.
To expand on why is that not needed, but feel free to dispute
- addresses all pre-validated
- there is no possible gadget here, it's a write, writes are not
speculated
- lock prefix so no speculation anyway
> 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
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH RFC] vhost: set dirty log bits without pinning pages on x86
2026-11-13 1:29 ` zhaoguohan
2026-11-13 5:47 ` Michael S. Tsirkin
2026-11-13 6:09 ` Michael S. Tsirkin
@ 2026-11-13 13:51 ` Michael S. Tsirkin
2 siblings, 0 replies; 6+ messages in thread
From: Michael S. Tsirkin @ 2026-11-13 13:51 UTC (permalink / raw)
To: zhaoguohan
Cc: bp, dave.hansen, eperezma, hpa, jasowang, kvm, linux-kernel,
mingo, netdev, tglx, virtualization, x86
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 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. 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
So, you were right, it's better to check access. But I don't like
paying nospec. Let's use the modern scoped accessors?
Like this maybe? Very lightly tested, no time today, sorry.
If you want to use it:
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 3a0dd3c2b233e..0bffa43e02179 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -594,6 +594,28 @@ _label: \
__ret; \
})
+/* set_bit() on a byte of user memory; returns -EFAULT on fault. */
+#define set_bit_user(nr, addr) \
+({ \
+ int __err = -EFAULT; \
+ u8 __user *__p = (addr); \
+ scoped_user_rw_access(__p, efault) { \
+ __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"); \
+ } \
+efault: \
+ __err; \
+})
+
/*
* We want the unsafe accessors to always be inlined and use
* the error labels - thus the macro games.
^ permalink raw reply related [flat|nested] 6+ messages in thread