From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Andrea Arcangeli <aarcange@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,
Jerome Glisse <jglisse@redhat.com>
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address
Date: Mon, 11 Mar 2019 23:50:47 -0400 [thread overview]
Message-ID: <20190311234956-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <ff45ea43-1145-5ea6-767c-1a99d55a9c61@redhat.com>
On Tue, Mar 12, 2019 at 10:52:15AM +0800, Jason Wang wrote:
>
> On 2019/3/11 下午8:48, Michael S. Tsirkin wrote:
> > On Mon, Mar 11, 2019 at 03:40:31PM +0800, Jason Wang wrote:
> > > On 2019/3/9 上午3:48, Andrea Arcangeli wrote:
> > > > Hello Jeson,
> > > >
> > > > On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote:
> > > > > Just to make sure I understand here. For boosting through huge TLB, do
> > > > > you mean we can do that in the future (e.g by mapping more userspace
> > > > > pages to kenrel) or it can be done by this series (only about three 4K
> > > > > pages were vmapped per virtqueue)?
> > > > When I answered about the advantages of mmu notifier and I mentioned
> > > > guaranteed 2m/gigapages where available, I overlooked the detail you
> > > > were using vmap instead of kmap. So with vmap you're actually doing
> > > > the opposite, it slows down the access because it will always use a 4k
> > > > TLB even if QEMU runs on THP or gigapages hugetlbfs.
> > > >
> > > > If there's just one page (or a few pages) in each vmap there's no need
> > > > of vmap, the linearity vmap provides doesn't pay off in such
> > > > case.
> > > >
> > > > So likely there's further room for improvement here that you can
> > > > achieve in the current series by just dropping vmap/vunmap.
> > > >
> > > > You can just use kmap (or kmap_atomic if you're in preemptible
> > > > section, should work from bh/irq).
> > > >
> > > > In short the mmu notifier to invalidate only sets a "struct page *
> > > > userringpage" pointer to NULL without calls to vunmap.
> > > >
> > > > In all cases immediately after gup_fast returns you can always call
> > > > put_page immediately (which explains why I'd like an option to drop
> > > > FOLL_GET from gup_fast to speed it up).
> > > >
> > > > Then you can check the sequence_counter and inc/dec counter increased
> > > > by _start/_end. That will tell you if the page you got and you called
> > > > put_page to immediately unpin it or even to free it, cannot go away
> > > > under you until the invalidate is called.
> > > >
> > > > If sequence counters and counter tells that gup_fast raced with anyt
> > > > mmu notifier invalidate you can just repeat gup_fast. Otherwise you're
> > > > done, the page cannot go away under you, the host virtual to host
> > > > physical mapping cannot change either. And the page is not pinned
> > > > either. So you can just set the "struct page * userringpage = page"
> > > > where "page" was the one setup by gup_fast.
> > > >
> > > > When later the invalidate runs, you can just call set_page_dirty if
> > > > gup_fast was called with "write = 1" and then you clear the pointer
> > > > "userringpage = NULL".
> > > >
> > > > When you need to read/write to the memory
> > > > kmap/kmap_atomic(userringpage) should work.
> > > Yes, I've considered kmap() from the start. The reason I don't do that is
> > > large virtqueue may need more than one page so VA might not be contiguous.
> > > But this is probably not a big issue which just need more tricks in the
> > > vhost memory accessors.
> > >
> > >
> > > > In short because there's no hardware involvement here, the established
> > > > mapping is just the pointer to the page, there is no need of setting
> > > > up any pagetables or to do any TLB flushes (except on 32bit archs if
> > > > the page is above the direct mapping but it never happens on 64bit
> > > > archs).
> > > I see, I believe we don't care much about the performance of 32bit archs (or
> > > we can just fallback to copy_to_user() friends).
> > Using copyXuser is better I guess.
>
>
> Ok.
>
>
> >
> > > Using direct mapping (I
> > > guess kernel will always try hugepage for that?) should be better and we can
> > > even use it for the data transfer not only for the metadata.
> > >
> > > Thanks
> > We can't really. The big issue is get user pages. Doing that on data
> > path will be slower than copyXuser.
>
>
> I meant if we can find a way to avoid doing gup in datapath. E.g vhost
> maintain a range tree and add or remove ranges through MMU notifier. Then in
> datapath, if we find the range, then use direct mapping otherwise
> copy_to_user().
>
> Thanks
We can try. But I'm not sure there's any reason to think there's any
locality there.
>
> > Or maybe it won't with the
> > amount of mitigations spread around. Go ahead and try.
> >
> >
next prev parent reply other threads:[~2019-03-12 3:50 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 [this message]
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
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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