From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42214) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dfAnK-0002W6-HT for qemu-devel@nongnu.org; Tue, 08 Aug 2017 16:08:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dfAnF-0007iB-IC for qemu-devel@nongnu.org; Tue, 08 Aug 2017 16:08:02 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48166) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dfAnF-0007hk-C7 for qemu-devel@nongnu.org; Tue, 08 Aug 2017 16:07:57 -0400 Date: Tue, 8 Aug 2017 23:07:49 +0300 From: "Michael S. Tsirkin" Message-ID: <20170808230043-mutt-send-email-mst@kernel.org> References: <859362e8-0d98-3865-8bad-a15bfa218167@redhat.com> <20170726092931.0678689e@w520.home> <20170726190348-mutt-send-email-mst@kernel.org> <20170726113222.52aad9a6@w520.home> <20170731234626.7664be18@w520.home> <20170801090158.35d18f10@w520.home> <20170807095224.5438ef8c@w520.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170807095224.5438ef8c@w520.home> Subject: Re: [Qemu-devel] =?utf-8?q?About_virtio_device_hotplug_in_Q35!_?= =?utf-8?b?44CQ5aSW5Z+f6YKu5Lu2LuiwqOaFjuafpemYheOAkQ==?= List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex Williamson Cc: Bob Chen , Marcel Apfelbaum , =?utf-8?B?6ZmI5Y2a?= , qemu-devel@nongnu.org On Mon, Aug 07, 2017 at 09:52:24AM -0600, Alex Williamson wrote: > I wonder if it has something to do > with the link speed/width advertised on the switch port. I don't think > the endpoint can actually downshift the physical link, so lspci on the > host should probably still show the full bandwidth capability, but > maybe the driver is somehow doing rate limiting. PCIe gets a little > more complicated as we go to newer versions, so it's not quite as > simple as exposing a different bit configuration to advertise 8GT/s, > x16. Last I tried to do link matching it was deemed too complicated > for something I couldn't prove at the time had measurable value. This > might be a good way to prove that value if it makes a difference here. > I can't think why else you'd see such a performance difference, but > testing to see if the KVM exit rate is significantly different could > still be an interesting verification. It might be easiest to just dust off that patch and see whether it helps. -- MST