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[209.132.183.28]) by mx.google.com with ESMTPS id z6si546853qke.0.2019.03.11.19.52.27 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Mar 2019 19:52:27 -0700 (PDT) Received-SPF: pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jasowang@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jasowang@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D6559C049E23; Tue, 12 Mar 2019 02:52:26 +0000 (UTC) Received: from [10.72.12.17] (ovpn-12-17.pek2.redhat.com [10.72.12.17]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3C3EB5DA27; Tue, 12 Mar 2019 02:52:16 +0000 (UTC) Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address To: "Michael S. Tsirkin" Cc: Andrea Arcangeli , 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 References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190307103503-mutt-send-email-mst@kernel.org> <20190307124700-mutt-send-email-mst@kernel.org> <20190307191622.GP23850@redhat.com> <20190308194845.GC26923@redhat.com> <8b68a2a0-907a-15f5-a07f-fc5b53d7ea19@redhat.com> <20190311084525-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Tue, 12 Mar 2019 10:52:15 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190311084525-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Tue, 12 Mar 2019 02:52:26 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > Or maybe it won't with the > amount of mitigations spread around. Go ahead and try. > >