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Tsirkin" To: zhaoguohan@kylinos.cn Cc: jasowangio@gmail.com, eperezma@redhat.com, xuanzhuo@linux.alibaba.com, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] vhost: set dirty log bits without pinning pages on x86 Message-ID: <20261112040926-mutt-send-email-mst@kernel.org> References: <20261112085631.128819-1-zhaoguohan@kylinos.cn> <20261112085631.128819-2-zhaoguohan@kylinos.cn> X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261112085631.128819-2-zhaoguohan@kylinos.cn> On Thu, Nov 12, 2026 at 04:56:30PM +0800, zhaoguohan@kylinos.cn wrote: > From: Khan Zhao > > set_bit_to_user() pins the log page with pin_user_pages_fast(), maps it > with kmap_atomic(), sets one bit and then unpins the page and marks it > dirty. This runs for every bit set in the dirty log, e.g. on each used > ring update while logging is enabled for live migration. > > log_write() always passes a byte address and a bit number in [0, 7], so > the update can be done as a byte-sized cmpxchg on the user address. On > x86, try that first using unsafe_try_cmpxchg_user() inside a > user_access_begin()/user_access_end() section with page faults disabled. > If the access faults (page not present, read-only or needing COW), or > the cmpxchg loses to concurrent updates of the same byte four times in > a row, fall back to the existing pin/kmap/set_bit path, which can fault > the page in. > > unsafe_try_cmpxchg_user() is only implemented on x86, so other > architectures keep using the existing path. > > On an x86_64 KVM guest (i7-12800HX host), a synthetic used-ring > workload with logging enabled went from 243.60 to 171.11 ns per > descriptor, averaged over five alternating runs. This is the > end-to-end cost per descriptor, not live migration performance. > > Signed-off-by: Khan Zhao > --- > drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++++----- > 1 file changed, 45 insertions(+), 5 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index e29f154706da..9c3045504709 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2527,11 +2527,7 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp) > } > EXPORT_SYMBOL_GPL(vhost_dev_ioctl); > > -/* 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. > - */ > -static int set_bit_to_user(int nr, void __user *addr) > +static int set_bit_to_user_slow(int nr, void __user *addr) > { > unsigned long log = (unsigned long)addr; > struct page *page; > @@ -2550,6 +2546,50 @@ static int set_bit_to_user(int nr, void __user *addr) > return 0; > } > > +#ifdef CONFIG_X86 > +static bool set_bit_to_user_fast(int nr, void __user *addr) > +{ > + u8 __user *p = addr; > + u8 old; > + int retries = 4; > + bool success = false; > + > + /* > + * log_write() passes a byte address and a bit number in [0, 7]. > + * On x86, update just that byte to avoid crossing a page boundary. > + * Bound retries so a contended byte cannot hold up the worker. > + */ > + pagefault_disable(); > + if (!user_access_begin(p, sizeof(*p))) > + goto out; > + unsafe_get_user(old, p, fault); > + do { > + if (unsafe_try_cmpxchg_user(p, &old, old | BIT(nr), fault)) { > + success = true; > + break; > + } > + } while (--retries); > +fault: > + user_access_end(); > +out: > + pagefault_enable(); > + return success; > +} Yes we wanted this for the longest time! But can we use the nice scoped_user_rw_access API? Something like: pagefault_disable(); scoped_user_rw_access(p, efault) { unsafe_get_user(old, p, efault); do { if (unsafe_try_cmpxchg_user(p, &old, old | BIT(nr), efault)) { success = true; break; } } while (--retries); } pagefault_enable(); return success; efault: pagefault_enable(); return false; maybe? > +#else > +static bool set_bit_to_user_fast(int nr, void __user *addr) > +{ > + return false; > +} > +#endif /* CONFIG_X86 */ > + > +static int set_bit_to_user(int nr, void __user *addr) > +{ > + if (set_bit_to_user_fast(nr, addr)) > + return 0; > + /* Resolve faults, or use the native bit layout on other architectures. */ > + return set_bit_to_user_slow(nr, addr); > +} > + > static int log_write(void __user *log_base, > u64 write_address, u64 write_length) > { > -- > 2.43.0