Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759748Ab3DDMWm (ORCPT ); Thu, 4 Apr 2013 08:22:42 -0400 Received: from cantor2.suse.de ([195.135.220.15]:38129 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759504Ab3DDMWl convert rfc822-to-8bit (ORCPT ); Thu, 4 Apr 2013 08:22:41 -0400 Subject: Re: [PATCH RFC] kvm: add PV MMIO EVENTFD Mime-Version: 1.0 (Apple Message framework v1278) Content-Type: text/plain; charset=us-ascii From: Alexander Graf In-Reply-To: <20130404121934.GE17919@redhat.com> Date: Thu, 4 Apr 2013 14:22:39 +0200 Cc: "Michael S. Tsirkin" , Marcelo Tosatti , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, Xiao Guangrong , Takuya Yoshikawa , Alex Williamson , Will Deacon , Christoffer Dall , Sasha Levin , Andrew Morton , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org Content-Transfer-Encoding: 8BIT Message-Id: References: <20130404104846.GA6227@redhat.com> <6D40C5F1-849E-4F32-B391-E1A1BFA8C59D@suse.de> <20130404110442.GA6467@redhat.com> <20130404121934.GE17919@redhat.com> To: Gleb Natapov X-Mailer: Apple Mail (2.1278) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2483 Lines: 72 On 04.04.2013, at 14:19, Gleb Natapov wrote: > On Thu, Apr 04, 2013 at 02:09:53PM +0200, Alexander Graf wrote: >> >> On 04.04.2013, at 13:04, Michael S. Tsirkin wrote: >> >>> On Thu, Apr 04, 2013 at 01:57:34PM +0200, Alexander Graf wrote: >>>> >>>> On 04.04.2013, at 12:50, Michael S. Tsirkin wrote: >>>> >>>>> With KVM, MMIO is much slower than PIO, due to the need to >>>>> do page walk and emulation. But with EPT, it does not have to be: we >>>>> know the address from the VMCS so if the address is unique, we can look >>>>> up the eventfd directly, bypassing emulation. >>>>> >>>>> Add an interface for userspace to specify this per-address, we can >>>>> use this e.g. for virtio. >>>>> >>>>> The implementation adds a separate bus internally. This serves two >>>>> purposes: >>>>> - minimize overhead for old userspace that does not use PV MMIO >>>>> - minimize disruption in other code (since we don't know the length, >>>>> devices on the MMIO bus only get a valid address in write, this >>>>> way we don't need to touch all devices to teach them handle >>>>> an dinvalid length) >>>>> >>>>> At the moment, this optimization is only supported for EPT on x86 and >>>>> silently ignored for NPT and MMU, so everything works correctly but >>>>> slowly. >>>>> >>>>> TODO: NPT, MMU and non x86 architectures. >>>>> >>>>> The idea was suggested by Peter Anvin. Lots of thanks to Gleb for >>>>> pre-review and suggestions. >>>>> >>>>> Signed-off-by: Michael S. Tsirkin >>>> >>>> This still uses page fault intercepts which are orders of magnitudes >>>> slower than hypercalls. >>> >>> Not really. Here's a test: >>> compare vmcall to portio: >>> >>> vmcall 1519 >>> ... >>> outl_to_kernel 1745 >>> >>> compare portio to mmio: >>> >>> mmio-wildcard-eventfd:pci-mem 3529 >>> mmio-pv-eventfd:pci-mem 1878 >>> portio-wildcard-eventfd:pci-io 1846 >>> >>> So not orders of magnitude. >> >> https://dl.dropbox.com/u/8976842/KVM%20Forum%202012/MMIO%20Tuning.pdf >> >> Check out page 41. Higher is better (number is number of loop cycles in a second). My test system was an AMD Istanbul based box. >> > Have you bypassed instruction emulation in your testing? PIO doesn't do instruction emulation. Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/