From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: aarcange@redhat.com, linux-parisc@vger.kernel.org,
kvm@vger.kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org, hch@infradead.org,
linux-mm@kvack.org, David Miller <davem@davemloft.net>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap()
Date: Mon, 11 Mar 2019 23:52:41 -0400 [thread overview]
Message-ID: <20190311235140-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <6b6dcc4a-2f08-ba67-0423-35787f3b966c@redhat.com>
On Tue, Mar 12, 2019 at 10:59:09AM +0800, Jason Wang wrote:
>
> On 2019/3/12 上午2:14, David Miller wrote:
> > From: "Michael S. Tsirkin" <mst@redhat.com>
> > Date: Mon, 11 Mar 2019 09:59:28 -0400
> >
> > > On Mon, Mar 11, 2019 at 03:13:17PM +0800, Jason Wang wrote:
> > > > On 2019/3/8 下午10:12, Christoph Hellwig wrote:
> > > > > On Wed, Mar 06, 2019 at 02:18:07AM -0500, 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. This is done through setup kernel address through vmap() and
> > > > > > resigter MMU notifier for invalidation.
> > > > > >
> > > > > > Test shows about 24% improvement on TX PPS. TCP_STREAM doesn't see
> > > > > > obvious improvement.
> > > > > How is this going to work for CPUs with virtually tagged caches?
> > > >
> > > > Anything different that you worry?
> > > If caches have virtual tags then kernel and userspace view of memory
> > > might not be automatically in sync if they access memory
> > > through different virtual addresses. You need to do things like
> > > flush_cache_page, probably multiple times.
> > "flush_dcache_page()"
>
>
> I get this. Then I think the current set_bit_to_user() is suspicious, we
> probably miss a flush_dcache_page() there:
>
>
> static int set_bit_to_user(int nr, void __user *addr)
> {
> unsigned long log = (unsigned long)addr;
> struct page *page;
> void *base;
> int bit = nr + (log % PAGE_SIZE) * 8;
> int r;
>
> r = get_user_pages_fast(log, 1, 1, &page);
> if (r < 0)
> return r;
> BUG_ON(r != 1);
> base = kmap_atomic(page);
> set_bit(bit, base);
> kunmap_atomic(base);
> set_page_dirty_lock(page);
> put_page(page);
> return 0;
> }
>
> Thanks
I think you are right. The correct fix though is to re-implement
it using asm and handling pagefault, not gup.
Three atomic ops per bit is way to expensive.
--
MST
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next prev parent reply other threads:[~2019-03-12 3:52 UTC|newest]
Thread overview: 70+ 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 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 7:18 ` [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Jason Wang
[not found] ` <1551856692-3384-3-git-send-email-jasowang@redhat.com>
2019-03-06 10:45 ` [RFC PATCH V2 2/5] vhost: fine grain userspace memory accessors Christophe de Dinechin
[not found] ` <4C1386C5-F153-43DD-8B14-CC752FA5A07A@dinechin.org>
2019-03-07 2:38 ` Jason Wang
[not found] ` <1551856692-3384-5-git-send-email-jasowang@redhat.com>
2019-03-06 10:56 ` [RFC PATCH V2 4/5] vhost: introduce helpers to get the size of metadata area Christophe de Dinechin
[not found] ` <608E47C2-5130-41DE-9D52-02807EBCDD43@dinechin.org>
2019-03-07 2:40 ` Jason Wang
[not found] ` <CAFqt6zYynfCn_SG6w98dHMA5rS6euPb+ihXtQcALE47K0LQb7g@mail.gmail.com>
2019-03-07 2:42 ` Jason Wang
[not found] ` <1551856692-3384-6-git-send-email-jasowang@redhat.com>
2019-03-06 16:31 ` [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Michael S. Tsirkin
[not found] ` <20190306092837-mutt-send-email-mst@kernel.org>
2019-03-07 2:45 ` Jason Wang
[not found] ` <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com>
2019-03-07 15:34 ` Michael S. Tsirkin
[not found] ` <20190307101708-mutt-send-email-mst@kernel.org>
2019-03-07 19:09 ` Jerome Glisse
[not found] ` <20190307190910.GE3835@redhat.com>
2019-03-07 19:38 ` Andrea Arcangeli
2019-03-07 20:17 ` Jerome Glisse
[not found] ` <20190307201722.GG3835@redhat.com>
2019-03-07 21:27 ` Andrea Arcangeli
[not found] ` <20190307212717.GS23850@redhat.com>
2019-03-08 9:13 ` Jason Wang
[not found] ` <671c4a98-4699-836e-79fc-0ce650c7f701@redhat.com>
2019-03-08 19:11 ` Andrea Arcangeli
[not found] ` <20190308191108.GA26923@redhat.com>
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
[not found] ` <20190307103503-mutt-send-email-mst@kernel.org>
2019-03-07 17:56 ` Michael S. Tsirkin
[not found] ` <20190307124700-mutt-send-email-mst@kernel.org>
2019-03-07 19:16 ` Andrea Arcangeli
2019-03-07 19:17 ` Jerome Glisse
[not found] ` <20190307191720.GF3835@redhat.com>
2019-03-08 2:21 ` Michael S. Tsirkin
2019-03-08 2:55 ` Jerome Glisse
[not found] ` <20190308025539.GA5562@redhat.com>
2019-03-08 3:16 ` Michael S. Tsirkin
[not found] ` <20190307221549-mutt-send-email-mst@kernel.org>
2019-03-08 3:40 ` Jerome Glisse
[not found] ` <20190308034053.GB5562@redhat.com>
2019-03-08 3:43 ` Michael S. Tsirkin
[not found] ` <20190307224143-mutt-send-email-mst@kernel.org>
2019-03-08 3:45 ` Jerome Glisse
[not found] ` <20190308034540.GC5562@redhat.com>
2019-03-08 9:15 ` Jason Wang
2019-03-08 8:58 ` Jason Wang
[not found] ` <43408100-84d9-a359-3e78-dc65fb7b0ad1@redhat.com>
2019-03-08 12:56 ` Michael S. Tsirkin
[not found] ` <20190308075506-mutt-send-email-mst@kernel.org>
2019-03-08 15:02 ` Jerome Glisse
2019-03-08 19:13 ` Andrea Arcangeli
[not found] ` <20190307191622.GP23850@redhat.com>
2019-03-08 8:50 ` Jason Wang
2019-03-08 14:58 ` Jerome Glisse
2019-03-08 19:48 ` Andrea Arcangeli
[not found] ` <20190308145800.GA3661@redhat.com>
2019-03-11 7:18 ` Jason Wang
[not found] ` <20190308194845.GC26923@redhat.com>
2019-03-08 20:06 ` Jerome Glisse
2019-03-11 7:40 ` Jason Wang
2019-03-11 12:48 ` Michael S. Tsirkin
[not found] ` <20190311084525-mutt-send-email-mst@kernel.org>
2019-03-11 13:43 ` Andrea Arcangeli
2019-03-12 2:52 ` Jason Wang
2019-03-12 3:50 ` Michael S. Tsirkin
[not found] ` <20190311234956-mutt-send-email-mst@kernel.org>
2019-03-12 7:15 ` Jason Wang
[not found] ` <20190311134305.GC23321@redhat.com>
2019-03-12 2:56 ` Jason Wang
[not found] ` <4979eed5-9e3f-5ee0-f4f4-1a5e2a839b21@redhat.com>
2019-03-12 3:51 ` Michael S. Tsirkin
2019-03-08 14:12 ` [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Christoph Hellwig
[not found] ` <20190308141220.GA21082@infradead.org>
2019-03-11 7:13 ` Jason Wang
[not found] ` <56374231-7ba7-0227-8d6d-4d968d71b4d6@redhat.com>
2019-03-11 13:59 ` Michael S. Tsirkin
[not found] ` <20190311095405-mutt-send-email-mst@kernel.org>
2019-03-11 18:14 ` David Miller
[not found] ` <20190311.111413.1140896328197448401.davem@davemloft.net>
2019-03-12 2:59 ` Jason Wang
[not found] ` <6b6dcc4a-2f08-ba67-0423-35787f3b966c@redhat.com>
2019-03-12 3:52 ` Michael S. Tsirkin [this message]
2019-03-12 7:17 ` Jason Wang
[not found] ` <76c353ed-d6de-99a9-76f9-f258074c1462@redhat.com>
2019-03-12 11:54 ` Michael S. Tsirkin
[not found] ` <20190312075033-mutt-send-email-mst@kernel.org>
[not found] ` <1552405610.3083.17.camel@HansenPartnership.com>
2019-03-12 20:04 ` Andrea Arcangeli
[not found] ` <1552424017.14432.11.camel@HansenPartnership.com>
2019-03-12 21:11 ` Andrea Arcangeli
[not found] ` <20190312211117.GB25147@redhat.com>
[not found] ` <1552425555.14432.14.camel@HansenPartnership.com>
2019-03-12 21:53 ` Andrea Arcangeli
[not found] ` <20190312215321.GC25147@redhat.com>
[not found] ` <1552428174.14432.39.camel@HansenPartnership.com>
2019-03-12 22:50 ` Andrea Arcangeli
2019-03-13 16:05 ` Christoph Hellwig
[not found] ` <20190313160529.GB15134@infradead.org>
[not found] ` <1552495028.3022.37.camel@HansenPartnership.com>
2019-03-14 10:42 ` Michael S. Tsirkin
[not found] ` <20190314064004-mutt-send-email-mst@kernel.org>
2019-03-14 13:49 ` Jason Wang
[not found] ` <74015196-2e18-4fe0-50ac-0c9d497315c7@redhat.com>
2019-03-14 19:33 ` Andrea Arcangeli
[not found] ` <20190314193333.GA24542@redhat.com>
2019-03-15 4:39 ` Jason Wang
[not found] ` <1552367685.23859.22.camel@HansenPartnership.com>
2019-03-12 7:51 ` Jason Wang
[not found] ` <f9e52313-0a06-22b6-140c-ded75eecde20@redhat.com>
2019-03-12 7:53 ` Jason Wang
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