From: Jerome Glisse <jglisse@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>,
kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
peterx@redhat.com, linux-mm@kvack.org, aarcange@redhat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
Date: Thu, 7 Mar 2019 22:45:40 -0500 [thread overview]
Message-ID: <20190308034540.GC5562@redhat.com> (raw)
In-Reply-To: <20190307224143-mutt-send-email-mst@kernel.org>
On Thu, Mar 07, 2019 at 10:43:12PM -0500, Michael S. Tsirkin wrote:
> On Thu, Mar 07, 2019 at 10:40:53PM -0500, Jerome Glisse wrote:
> > On Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote:
> > > On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote:
> > > > On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S. Tsirkin wrote:
> > > > > On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse wrote:
> > > > > > > It's because of all these issues that I preferred just accessing
> > > > > > > userspace memory and handling faults. Unfortunately there does not
> > > > > > > appear to exist an API that whitelists a specific driver along the lines
> > > > > > > of "I checked this code for speculative info leaks, don't add barriers
> > > > > > > on data path please".
> > > > > >
> > > > > > Maybe it would be better to explore adding such helper then remapping
> > > > > > page into kernel address space ?
> > > > >
> > > > > I explored it a bit (see e.g. thread around: "__get_user slower than
> > > > > get_user") and I can tell you it's not trivial given the issue is around
> > > > > security. So in practice it does not seem fair to keep a significant
> > > > > optimization out of kernel because *maybe* we can do it differently even
> > > > > better :)
> > > >
> > > > Maybe a slightly different approach between this patchset and other
> > > > copy user API would work here. What you want really is something like
> > > > a temporary mlock on a range of memory so that it is safe for the
> > > > kernel to access range of userspace virtual address ie page are
> > > > present and with proper permission hence there can be no page fault
> > > > while you are accessing thing from kernel context.
> > > >
> > > > So you can have like a range structure and mmu notifier. When you
> > > > lock the range you block mmu notifier to allow your code to work on
> > > > the userspace VA safely. Once you are done you unlock and let the
> > > > mmu notifier go on. It is pretty much exactly this patchset except
> > > > that you remove all the kernel vmap code. A nice thing about that
> > > > is that you do not need to worry about calling set page dirty it
> > > > will already be handle by the userspace VA pte. It also use less
> > > > memory than when you have kernel vmap.
> > > >
> > > > This idea might be defeated by security feature where the kernel is
> > > > running in its own address space without the userspace address
> > > > space present.
> > >
> > > Like smap?
> >
> > Yes like smap but also other newer changes, with similar effect, since
> > the spectre drama.
> >
> > Cheers,
> > Jérôme
>
> Sorry do you mean meltdown and kpti?
Yes all that and similar thing. I do not have the full list in my head.
Cheers,
Jérôme
next prev parent reply other threads:[~2019-03-08 3:45 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-06 7:18 [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 1/5] vhost: generalize adding used elem Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 2/5] vhost: fine grain userspace memory accessors Jason Wang
2019-03-06 10:45 ` Christophe de Dinechin
2019-03-07 2:38 ` Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 3/5] vhost: rename vq_iotlb_prefetch() to vq_meta_prefetch() Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 4/5] vhost: introduce helpers to get the size of metadata area Jason Wang
2019-03-06 10:56 ` Christophe de Dinechin
2019-03-07 2:40 ` Jason Wang
2019-03-06 18:43 ` Souptick Joarder
2019-03-07 2:42 ` Jason Wang
2019-03-06 7:18 ` [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Jason Wang
2019-03-06 16:31 ` Michael S. Tsirkin
2019-03-07 2:45 ` Jason Wang
2019-03-07 15:34 ` Michael S. Tsirkin
2019-03-07 19:09 ` Jerome Glisse
2019-03-07 19:38 ` Andrea Arcangeli
2019-03-07 20:17 ` Jerome Glisse
2019-03-07 21:27 ` Andrea Arcangeli
2019-03-08 9:13 ` Jason Wang
2019-03-08 19:11 ` Andrea Arcangeli
2019-03-11 7:21 ` Jason Wang
2019-03-11 14:45 ` Jan Kara
2019-03-08 8:31 ` Jason Wang
2019-03-07 15:47 ` Michael S. Tsirkin
2019-03-07 17:56 ` Michael S. Tsirkin
2019-03-07 19:16 ` Andrea Arcangeli
2019-03-08 8:50 ` Jason Wang
2019-03-08 14:58 ` Jerome Glisse
2019-03-11 7:18 ` Jason Wang
2019-03-08 19:48 ` Andrea Arcangeli
2019-03-08 20:06 ` Jerome Glisse
2019-03-11 7:40 ` Jason Wang
2019-03-11 12:48 ` Michael S. Tsirkin
2019-03-11 13:43 ` Andrea Arcangeli
2019-03-12 2:56 ` Jason Wang
2019-03-12 3:51 ` Michael S. Tsirkin
2019-03-12 2:52 ` Jason Wang
2019-03-12 3:50 ` Michael S. Tsirkin
2019-03-12 7:15 ` Jason Wang
2019-03-07 19:17 ` Jerome Glisse
2019-03-08 2:21 ` Michael S. Tsirkin
2019-03-08 2:55 ` Jerome Glisse
2019-03-08 3:16 ` Michael S. Tsirkin
2019-03-08 3:40 ` Jerome Glisse
2019-03-08 3:43 ` Michael S. Tsirkin
2019-03-08 3:45 ` Jerome Glisse [this message]
2019-03-08 9:15 ` Jason Wang
2019-03-08 8:58 ` Jason Wang
2019-03-08 12:56 ` Michael S. Tsirkin
2019-03-08 15:02 ` Jerome Glisse
2019-03-08 19:13 ` Andrea Arcangeli
2019-03-08 14:12 ` [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Christoph Hellwig
2019-03-11 7:13 ` Jason Wang
2019-03-11 13:59 ` Michael S. Tsirkin
2019-03-11 18:14 ` David Miller
2019-03-12 2:59 ` Jason Wang
2019-03-12 3:52 ` Michael S. Tsirkin
2019-03-12 7:17 ` Jason Wang
2019-03-12 11:54 ` Michael S. Tsirkin
2019-03-12 15:46 ` James Bottomley
2019-03-12 20:04 ` Andrea Arcangeli
2019-03-12 20:53 ` James Bottomley
2019-03-12 21:11 ` Andrea Arcangeli
2019-03-12 21:19 ` James Bottomley
2019-03-12 21:53 ` Andrea Arcangeli
2019-03-12 22:02 ` James Bottomley
2019-03-12 22:50 ` Andrea Arcangeli
2019-03-12 22:57 ` James Bottomley
2019-03-13 16:05 ` Christoph Hellwig
2019-03-13 16:37 ` James Bottomley
2019-03-14 10:42 ` Michael S. Tsirkin
2019-03-14 13:49 ` Jason Wang
2019-03-14 19:33 ` Andrea Arcangeli
2019-03-15 4:39 ` Jason Wang
2019-03-12 5:14 ` James Bottomley
2019-03-12 7:51 ` Jason Wang
2019-03-12 7:53 ` Jason Wang
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