From: Dan Williams <dan.j.williams@intel.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>, KVM list <kvm@vger.kernel.org>,
virtualization@lists.linux-foundation.org,
Netdev <netdev@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
David Miller <davem@davemloft.net>
Subject: Re: [RFC PATCH V3 0/5] Hi:
Date: Sun, 6 Jan 2019 23:15:20 -0800 [thread overview]
Message-ID: <CAPcyv4iXFy0jfoQ_jQYq2yA7cqu4_Jcap4mvQesRRrV-hahGvQ@mail.gmail.com> (raw)
In-Reply-To: <20190106230224-mutt-send-email-mst@kernel.org>
On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote:
> >
> > On 2019/1/7 上午11:28, Michael S. Tsirkin wrote:
> > > On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
> > > > On 2019/1/3 上午4:47, Michael S. Tsirkin wrote:
> > > > > On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
> > > > > > This series tries to access virtqueue metadata through kernel virtual
> > > > > > address instead of copy_user() friends since they had too much
> > > > > > overheads like checks, spec barriers or even hardware feature
> > > > > > toggling.
> > > > > Will review, thanks!
> > > > > One questions that comes to mind is whether it's all about bypassing
> > > > > stac/clac. Could you please include a performance comparison with
> > > > > nosmap?
> > > > >
> > > > On machine without SMAP (Sandy Bridge):
> > > >
> > > > Before: 4.8Mpps
> > > >
> > > > After: 5.2Mpps
> > > OK so would you say it's really unsafe versus safe accesses?
> > > Or would you say it's just a better written code?
> >
> >
> > It's the effect of removing speculation barrier.
>
>
> You mean __uaccess_begin_nospec introduced by
> commit 304ec1b050310548db33063e567123fae8fd0301
> ?
>
> So fundamentally we do access_ok checks when supplying
> the memory table to the kernel thread, and we should
> do the spec barrier there.
>
> Then we can just create and use a variant of uaccess macros that does
> not include the barrier?
>
> Or, how about moving the barrier into access_ok?
> This way repeated accesses with a single access_ok get a bit faster.
> CC Dan Williams on this idea.
It would be interesting to see how expensive re-doing the address
limit check is compared to the speculation barrier. I.e. just switch
vhost_get_user() to use get_user() rather than __get_user(). That will
sanitize the pointer in the speculative path without a barrier.
I recall we had a convert access_ok() discussion with this result here:
https://lkml.org/lkml/2018/1/17/929
...but it sounds like you are proposing a smaller scope fixup for the
vhost use case? Something like barrier_nospec() in the success path
for all vhost access_ok() checks and then a get_user() variant that
disables the barrier.
next prev parent reply other threads:[~2019-01-07 7:15 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-29 12:46 [RFC PATCH V3 0/5] Hi: Jason Wang
2018-12-29 12:46 ` [RFC PATCH V3 1/5] vhost: generalize adding used elem Jason Wang
2019-01-04 21:29 ` Michael S. Tsirkin
2019-01-05 0:33 ` Sean Christopherson
2019-01-07 7:00 ` Jason Wang
2019-01-07 14:50 ` Michael S. Tsirkin
2018-12-29 12:46 ` [RFC PATCH V3 2/5] vhost: fine grain userspace memory accessors Jason Wang
2018-12-29 12:46 ` [RFC PATCH V3 3/5] vhost: rename vq_iotlb_prefetch() to vq_meta_prefetch() Jason Wang
2018-12-29 12:46 ` [RFC PATCH V3 4/5] vhost: introduce helpers to get the size of metadata area Jason Wang
2018-12-29 12:46 ` [RFC PATCH V3 5/5] vhost: access vq metadata through kernel virtual address Jason Wang
2019-01-04 21:34 ` Michael S. Tsirkin
2019-01-07 8:40 ` Jason Wang
2019-01-02 20:47 ` [RFC PATCH V3 0/5] Hi: Michael S. Tsirkin
2019-01-07 2:19 ` Jason Wang
2019-01-07 3:28 ` Michael S. Tsirkin
2019-01-07 3:53 ` Jason Wang
2019-01-07 4:17 ` Michael S. Tsirkin
2019-01-07 6:50 ` Jason Wang
2019-01-07 14:37 ` Michael S. Tsirkin
2019-01-08 10:01 ` Jason Wang
2019-01-07 7:15 ` Dan Williams [this message]
2019-01-07 14:11 ` Michael S. Tsirkin
2019-01-07 21:39 ` Dan Williams
2019-01-07 22:25 ` Michael S. Tsirkin
2019-01-07 22:44 ` Dan Williams
2019-01-09 4:31 ` __get_user slower than get_user (was Re: [RFC PATCH V3 0/5] Hi:) Michael S. Tsirkin
2019-01-09 5:19 ` Linus Torvalds
2019-01-08 11:42 ` [RFC PATCH V3 0/5] Hi: Jason Wang
2019-01-04 21:41 ` Michael S. Tsirkin
2019-01-07 6:58 ` Jason Wang
2019-01-07 14:47 ` Michael S. Tsirkin
2019-01-08 10:12 ` Jason Wang
2019-01-11 8:59 ` Jason Wang
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