linux-kernel - Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put

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Message-ID: <20251114185424.354133ae@pumpkin>
Date: Fri, 14 Nov 2025 18:54:24 +0000
From: David Laight <david.laight.linux@...il.com>
To: Jon Kohler <jon@...anix.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Eugenio Pérez <eperezma@...hat.com>, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Linus Torvalds
 <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>, Sean
 Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH net-next] vhost: use "checked" versions of get_user()
 and put_user()

On Wed, 12 Nov 2025 17:55:28 -0700
Jon Kohler <jon@...anix.com> wrote:

> vhost_get_user and vhost_put_user leverage __get_user and __put_user,
> respectively, which were both added in 2016 by commit 6b1e6cc7855b
> ("vhost: new device IOTLB API"). In a heavy UDP transmit workload on a
> vhost-net backed tap device, these functions showed up as ~11.6% of
> samples in a flamegraph of the underlying vhost worker thread.
> 
> Quoting Linus from [1]:
>     Anyway, every single __get_user() call I looked at looked like
>     historical garbage. [...] End result: I get the feeling that we
>     should just do a global search-and-replace of the __get_user/
>     __put_user users, replace them with plain get_user/put_user instead,
>     and then fix up any fallout (eg the coco code).
> 
> Switch to plain get_user/put_user in vhost, which results in a slight
> throughput speedup. get_user now about ~8.4% of samples in flamegraph.
> 
> Basic iperf3 test on a Intel 5416S CPU with Ubuntu 25.10 guest:
> TX: taskset -c 2 iperf3 -c <rx_ip> -t 60 -p 5200 -b 0 -u -i 5
> RX: taskset -c 2 iperf3 -s -p 5200 -D
> Before: 6.08 Gbits/sec
> After:  6.32 Gbits/sec
> 
> As to what drives the speedup, Sean's patch [2] explains:
> 	Use the normal, checked versions for get_user() and put_user() instead of
> 	the double-underscore versions that omit range checks, as the checked
> 	versions are actually measurably faster on modern CPUs (12%+ on Intel,
> 	25%+ on AMD).

Is there an associated access_ok() that can also be removed?

	David