From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Date: Tue, 12 Mar 2019 07:54:02 -0400 Message-ID: <20190312075033-mutt-send-email-mst@kernel.org> References: <20190308141220.GA21082@infradead.org> <56374231-7ba7-0227-8d6d-4d968d71b4d6@redhat.com> <20190311095405-mutt-send-email-mst@kernel.org> <20190311.111413.1140896328197448401.davem@davemloft.net> <6b6dcc4a-2f08-ba67-0423-35787f3b966c@redhat.com> <20190311235140-mutt-send-email-mst@kernel.org> <76c353ed-d6de-99a9-76f9-f258074c1462@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: David Miller , hch@infradead.org, 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, linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <76c353ed-d6de-99a9-76f9-f258074c1462@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Tue, Mar 12, 2019 at 03:17:00PM +0800, Jason Wang wrote: > > On 2019/3/12 上午11:52, Michael S. Tsirkin wrote: > > 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" > > > > 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. > > > I agree but it needs to introduce new helpers in asm  for all archs which is > not trivial. We can have a generic implementation using kmap. > At least for -stable, we need the flush? > > > > Three atomic ops per bit is way to expensive. > > > Yes. > > Thanks See James's reply - I stand corrected we do kunmap so no need to flush. -- MST