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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jerome Glisse <jglisse@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 22:43:12 -0500	[thread overview]
Message-ID: <20190307224143-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20190308034053.GB5562@redhat.com>

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?

-- 
MST

  reply	other threads:[~2019-03-08  3:43 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 [this message]
2019-03-08  3:45                   ` Jerome Glisse
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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