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[109.64.41.91]) by smtp.gmail.com with ESMTPSA id z2sm1536029wrs.87.2020.06.02.22.46.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Jun 2020 22:46:17 -0700 (PDT) Date: Wed, 3 Jun 2020 01:46:15 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: Al Viro , linux-kernel@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok() Message-ID: <20200603013600-mutt-send-email-mst@kernel.org> References: <20200602084257.134555-1-mst@redhat.com> <20200603014815.GR23230@ZenIV.linux.org.uk> <3358ae96-abb6-6be9-346a-0e971cb84dcd@redhat.com> <20200603041849.GT23230@ZenIV.linux.org.uk> <3e723db8-0d55-fae6-288e-9d95905592db@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3e723db8-0d55-fae6-288e-9d95905592db@redhat.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Jun 03, 2020 at 01:18:54PM +0800, Jason Wang wrote: > > On 2020/6/3 下午12:18, Al Viro wrote: > > On Wed, Jun 03, 2020 at 11:57:11AM +0800, Jason Wang wrote: > > > > > > How widely do you hope to stretch the user_access areas, anyway? > > > > > > To have best performance for small packets like 64B, if possible, we want to > > > disable STAC not only for the metadata access done by vhost accessors but > > > also the data access via iov iterator. > > If you want to try and convince Linus to go for that, make sure to Cc > > me on that thread. Always liked quality flame... > > > > The same goes for interval tree lookups with uaccess allowed. IOW, I _really_ > > doubt that it's a good idea. > > > I see. We are just seeking an approach to perform better in order to compete > with userspace dpdk backends. > > I tried another approach of using direct mapping + mmu notifier [1] but the > synchronization with MMU notifier is not easy to perform well. > > [1] https://patchwork.kernel.org/patch/11133009/ > > > > > > > > Incidentally, who had come up with the name __vhost_get_user? > > > > Makes for lovey WTF moment for readers - esp. in vhost_put_user()... > > > > > > I think the confusion comes since it does not accept userspace pointer (when > > > IOTLB is enabled). > > > > > > How about renaming it as vhost_read()/vhost_write() ? > > Huh? > > > > __vhost_get_user() is IOTLB remapping of userland pointer. It does not access > > userland memory. Neither for read, nor for write. It is used by vhost_get_user() > > and vhost_put_user(). > > > > Why would you want to rename it into vhost_read _or_ vhost_write, and in any case, > > how do you give one function two names? IDGI... > > > I get you know, I thought you're concerning the names of > vhost_get_user()/vhost_put_user() but actually __vhost_get_user(). > > Maybe something like __vhost_fetch_uaddr() is better. > > Thanks It's basically vhost_translate_uaddr isn't it? BTW now I re-read it I don't understand __vhost_get_user_slow: static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq, void __user *addr, unsigned int size, int type) { int ret; ret = translate_desc(vq, (u64)(uintptr_t)addr, size, vq->iotlb_iov, ARRAY_SIZE(vq->iotlb_iov), VHOST_ACCESS_RO); .. } how does this work? how can we cast a pointer to guest address without adding any offsets? > > >