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

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Message-ID: <20251114203238.51123933@pumpkin>
Date: Fri, 14 Nov 2025 20:32:38 +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"
 <kvm@...r.kernel.org>, "virtualization@...ts.linux.dev"
 <virtualization@...ts.linux.dev>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, "linux-kernel@...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 Fri, 14 Nov 2025 19:30:32 +0000
Jon Kohler <jon@...anix.com> wrote:

> > On Nov 14, 2025, at 1:54 PM, David Laight <david.laight.linux@...il.com> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  CAUTION: External Email
> > 
> > |-------------------------------------------------------------------!
> > 
> > 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  
> 
> Hey David - IIUC, the access_ok() for non-iotlb setups is done at
> initial setup time, not per event, see vhost_vring_set_addr and
> for the vhost net side see vhost_net_set_backend -> 
> vhost_vq_access_ok.

This is a long way away from the actual access....

The early 'sanity check' might be worth keeping, but the code has to
allow for the user access faulting (the application might unmap it).
But, in some sense, that early check is optimising for the user passing
in an invalid buffer - so not actually worth while,

> Will lean on MST/Jason to help sanity check my understanding.
> 
> In the iotlb case, that’s handled differently (Jason can speak to
> that side), but I dont think there is something we’d remove there?

Isn't the application side much the same?
(But I don't know what the code is doing...)

	David