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Tsirkin" , Jason Wang , 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, linux-arm-kernel@lists.infradead.org, linux-parisc@vger.kernel.org Subject: Re: [RFC PATCH V2 0/5] vhost: accelerate metadata access through vmap() Message-ID: <20190313160529.GB15134@infradead.org> References: <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> <20190312075033-mutt-send-email-mst@kernel.org> <1552405610.3083.17.camel@HansenPartnership.com> <20190312200450.GA25147@redhat.com> <1552424017.14432.11.camel@HansenPartnership.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1552424017.14432.11.camel@HansenPartnership.com> User-Agent: Mutt/1.9.2 (2017-12-15) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html 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 Tue, Mar 12, 2019 at 01:53:37PM -0700, James Bottomley wrote: > I've got to say: optimize what? What code do we ever have in the > kernel that kmap's a page and then doesn't do anything with it? You can > guarantee that on kunmap the page is either referenced (needs > invalidating) or updated (needs flushing). The in-kernel use of kmap is > always > > kmap > do something with the mapped page > kunmap > > In a very short interval. It seems just a simplification to make > kunmap do the flush if needed rather than try to have the users > remember. The thing which makes this really simple is that on most > architectures flush and invalidate is the same operation. If you > really want to optimize you can use the referenced and dirty bits on > the kmapped pte to tell you what operation to do, but if your flush is > your invalidate, you simply assume the data needs flushing on kunmap > without checking anything. I agree that this would be a good way to simplify the API. Now we'd just need volunteers to implement this for all architectures that need cache flushing and then remove the explicit flushing in the callers.. > > Which means after we fix vhost to add the flush_dcache_page after > > kunmap, Parisc will get a double hit (but it also means Parisc was > > the only one of those archs needed explicit cache flushes, where > > vhost worked correctly so far.. so it kinds of proofs your point of > > giving up being the safe choice). > > What double hit? If there's no cache to flush then cache flush is a > no-op. It's also a highly piplineable no-op because the CPU has the L1 > cache within easy reach. The only event when flush takes a large > amount time is if we actually have dirty data to write back to main > memory. I've heard people complaining that on some microarchitectures even no-op cache flushes are relatively expensive. Don't ask me why, but if we can easily avoid double flushes we should do that.