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[209.132.183.28]) by mx.google.com with ESMTPS id m4si390066qvg.167.2019.03.12.00.52.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Mar 2019 00:52:08 -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 4FA493082200; Tue, 12 Mar 2019 07:52:07 +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 07EDA5DD74; Tue, 12 Mar 2019 07:51:55 +0000 (UTC) Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() To: James Bottomley , David Miller , mst@redhat.com Cc: 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 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> <1552367685.23859.22.camel@HansenPartnership.com> From: Jason Wang Message-ID: Date: Tue, 12 Mar 2019 15:51:53 +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: <1552367685.23859.22.camel@HansenPartnership.com> 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 [10.5.110.47]); Tue, 12 Mar 2019 07:52:07 +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 2019/3/12 下午1:14, James Bottomley wrote: > On Tue, 2019-03-12 at 10:59 +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); > This sequence should be OK. get_user_pages() contains a flush which > clears the cache above the user virtual address, so on kmap, the page > is coherent at the new alias. On parisc at least, kunmap embodies a > flush_dcache_page() which pushes any changes in the cache above the > kernel virtual address back to main memory and makes it coherent again > for the user alias to pick it up. It would be good if kmap()/kunmap() can do this but looks like we can not assume this? For example, sparc's flush_dcache_page() doesn't do flush_dcache_page(). And bio_copy_data_iter() do flush_dcache_page() after kunmap_atomic(). Thanks > > James >