linux-kernel - Re: [PATCH 3/6] vhost: switch to use vhost_add_used

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Message-ID: <5212D61B.1010303@redhat.com>
Date:	Tue, 20 Aug 2013 10:36:11 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] vhost: switch to use vhost_add_used_n()

On 08/16/2013 05:56 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 16, 2013 at 01:16:27PM +0800, Jason Wang wrote:
>> > Let vhost_add_used() to use vhost_add_used_n() to reduce the code duplication.
>> > 
>> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> Does compiler inline it then?
> Reason I ask, last time I checked put_user inside vhost_add_used
> was much cheaper than copy_to_user inside vhost_add_used_n,
> so I wouldn't be surprised if this hurt performance.
> Did you check?
>

I run virtio_test but didn't see the difference.

Did you mean the might_fault() in __copy_to_user()? So how about switch
to use __put_user() if count is one in __vhost_add_used_n()?

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