Re: [PATCH 1/2] vhost: set dirty log bits without pinning pages on x86

[sashiko-bot@kernel.org]

> From: Khan Zhao <zhaoguohan@kylinos.cn>
> 
> 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 <zhaoguohan@kylinos.cn>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261112085631.128819-1-zhaoguohan@kylinos.cn?part=1