* [PATCH RFC] vhost: set dirty log bits without pinning pages on x86
@ 2026-11-12 10:53 Michael S. Tsirkin
2026-11-13 1:29 ` zhaoguohan
2026-11-13 10:54 ` sashiko-bot
0 siblings, 2 replies; 6+ messages in thread
From: Michael S. Tsirkin @ 2026-11-12 10:53 UTC (permalink / raw)
To: linux-kernel
Cc: Khan Zhao, Thomas Gleixner, Ingo Molnar, Borislav Petkov,
Dave Hansen, x86, H. Peter Anvin, Jason Wang, Eugenio Pérez,
kvm, virtualization, netdev
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.
Should not be too hard for non-x86 too.
Reported-by: Khan Zhao <zhaoguohan@kylinos.cn>
Link: https://lore.kernel.org/all/20261112085631.128819-1-zhaoguohan@kylinos.cn/
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
lightly tested. pls test and resubmit with your tool.
arch/x86/include/asm/uaccess.h | 18 ++++++++++++++++++
drivers/vhost/vhost.c | 7 +++++++
2 files changed, 25 insertions(+)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 3a0dd3c2b233e..ba1e7c3f83c8e 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -594,6 +594,24 @@ _label: \
__ret; \
})
+/* 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; \
+})
+
/*
* We want the unsafe accessors to always be inlined and use
* the error labels - thus the macro games.
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2f2c45d208832..9ba16cbd788cc 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2405,6 +2405,12 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
}
EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
+#ifdef CONFIG_X86
+static int set_bit_to_user(int nr, void __user *addr)
+{
+ return set_bit_user(nr, addr);
+}
+#else
/* TODO: This is really inefficient. We need something like get_user()
* (instruction directly accesses the data, with an exception table entry
* returning -EFAULT). See Documentation/arch/x86/exception-tables.rst.
@@ -2427,6 +2433,7 @@ static int set_bit_to_user(int nr, void __user *addr)
unpin_user_pages_dirty_lock(&page, 1, true);
return 0;
}
+#endif /* CONFIG_X86 */
static int log_write(void __user *log_base,
u64 write_address, u64 write_length)
--
MST
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH RFC] vhost: set dirty log bits without pinning pages on x86
2026-11-12 10:53 [PATCH RFC] vhost: set dirty log bits without pinning pages on x86 Michael S. Tsirkin
@ 2026-11-13 1:29 ` zhaoguohan
2026-11-13 5:47 ` Michael S. Tsirkin
` (2 more replies)
2026-11-13 10:54 ` sashiko-bot
1 sibling, 3 replies; 6+ messages in thread
From: zhaoguohan @ 2026-11-13 1:29 UTC (permalink / raw)
To: mst
Cc: bp, dave.hansen, eperezma, hpa, jasowang, kvm, linux-kernel,
mingo, netdev, tglx, virtualization, x86, zhaoguohan
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
^ 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 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
2026-11-12 10:53 [PATCH RFC] vhost: set dirty log bits without pinning pages on x86 Michael S. Tsirkin
2026-11-13 1:29 ` zhaoguohan
@ 2026-11-13 10:54 ` sashiko-bot
1 sibling, 0 replies; 6+ messages in thread
From: sashiko-bot @ 2026-11-13 10:54 UTC (permalink / raw)
To: Michael S. Tsirkin; +Cc: kvm, virtualization, Eugenio Perez
Thank you for your contribution! Sashiko AI review found 2 potential
issues to consider:
- [Critical] x86/uaccess, vhost: missing access_ok() check in
set_bit_user()
- [High] x86/uaccess: set_bit_user() uses write-only =r constraint on
__err
--
commit 508047a5899f1ab9b68b7a3a4c36fffabf6ab858
Author: Michael S. Tsirkin <mst@redhat.com>
vhost: set dirty log bits without pinning pages on x86
This commit introduces a set_bit_user() helper on x86 to atomically set a
bit in user memory using an exception table entry, and uses it in vhost's
set_bit_to_user() to avoid pinning user pages on the dirty-logging path.
Link: https://lore.kernel.org/all/20261112085631.128819-1-zhaoguohan@kylinos.cn/
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 3a0dd3c2b233e..ba1e7c3f83c8e 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -594,6 +594,24 @@ _label: \
> __ret; \
> })
>
> +/* 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; \
> +})
[Severity: Critical]
Does set_bit_user() need an access_ok() check (or user_access_begin() /
scoped_user_rw_access()) before calling __uaccess_begin()?
Previously, set_bit_to_user() called pin_user_pages_fast(), which rejected
kernel addresses. Not every vhost path validates vq->log_base with
access_ok() before reaching set_bit_to_user(): when vq->iotlb is non-NULL
and vq->log_used is set (with VHOST_F_LOG_ALL unset), vq_log_access_ok()
skips log_access_ok() because vq_log_used_access_ok() returns true early:
drivers/vhost/vhost.c:vq_log_used_access_ok() {
if (vq->iotlb)
return true;
return !log_used || log_access_ok(log_base, log_addr,
vhost_get_used_size(vq, vq->num));
}
When log_used() then runs, it calls log_write_hva() ->
log_write(vq->log_base, ...) -> set_bit_to_user(), allowing userspace to
set bits in arbitrary kernel memory with SMAP disabled.
[Severity: High]
Should the [errout] constraint on __err be "+r" instead of "=r"?
%[errout] is only written by the exception handler when instruction 1b
faults. With a write-only "=r" constraint, the compiler treats the
initial "__err = 0" assignment as dead and can reuse a register holding a
non-zero input (such as [mask]), causing set_bit_user() to return -EFAULT
even when the atomic orb succeeds and aborting vhost dirty-page logging.
--
Sashiko AI review · https://sashiko.dev/#/patchset/af7f05bf637337d35fdd10f3e0dbb5e39d7780e7.1794480795.git.mst@redhat.com?part=1
^ 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
end of thread, other threads:[~2026-11-13 13:51 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-11-12 10:53 [PATCH RFC] vhost: set dirty log bits without pinning pages on x86 Michael S. Tsirkin
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
2026-11-13 10:54 ` sashiko-bot
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