From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] Ideas wiki for GSoC 2010 Date: Mon, 15 Mar 2010 20:21:54 -0500 Message-ID: <4B9EDD32.10505@codemonkey.ws> References: <20100310183023.6632aece@redhat.com> <4B9E2745.7060903@redhat.com> <20100315125313.GK9457@il.ibm.com> <20100315130310.GE13108@8bytes.org> <4B9E320E.7040605@redhat.com> <4B9E34E1.3090709@codemonkey.ws> <4B9E4D11.70402@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Joerg Roedel , Muli Ben-Yehuda , Luiz Capitulino , qemu-devel@nongnu.org, aliguori@us.ibm.com, kvm@vger.kernel.org, jan.kiszka@siemens.com, agraf@suse.de, agl@us.ibm.com, Nadav Amit , Ben-Ami Yassour1 To: Avi Kivity Return-path: Received: from mail-gw0-f46.google.com ([74.125.83.46]:48520 "EHLO mail-gw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937321Ab0CPBV7 (ORCPT ); Mon, 15 Mar 2010 21:21:59 -0400 Received: by gwaa12 with SMTP id a12so124892gwa.19 for ; Mon, 15 Mar 2010 18:21:58 -0700 (PDT) In-Reply-To: <4B9E4D11.70402@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 03/15/2010 10:06 AM, Avi Kivity wrote: > On 03/15/2010 03:23 PM, Anthony Liguori wrote: >> On 03/15/2010 08:11 AM, Avi Kivity wrote: >>> On 03/15/2010 03:03 PM, Joerg Roedel wrote: >>>> >>>>>> I will add another project - iommu emulation. Could be very useful >>>>>> for doing device assignment to nested guests, which could make >>>>>> testing a lot easier. >>>>> Our experiments show that nested device assignment is pretty much >>>>> required for I/O performance in nested scenarios. >>>> Really? I did a small test with virtio-blk in a nested guest (disk >>>> read >>>> with dd, so not a real benchmark) and got a reasonable >>>> read-performance >>>> of around 25MB/s from the disk in the l2-guest. >>>> >>> >>> Your guest wasn't doing a zillion VMREADs and VMWRITEs every exit. >>> >>> I plan to reduce VMREAD/VMWRITE overhead for kvm, but not much we >>> can do for other guests. >> >> VMREAD/VMWRITEs are generally optimized by hypervisors as they tend >> to be costly. KVM is a bit unusual in terms of how many times the >> instructions are executed per exit. > > Do you know offhand of any unnecessary read/writes? There's > update_cr8_intercept(), but on normal exits, I don't see what else we > can remove. Yeah, there are a number of examples. vmcs_clear_bits() and vmcs_set_bits() read a field of the VMCS and then immediately writes it. This is unnecessary as the same information could be kept in a shadow variable. In vmx_fpu_activate, we call vmcs_clear_bits() followed immediately by vmcs_set_bits(). which means we're reading GUEST_CR0 twice and writing it twice. vmx_get_rflags() reads from the VMCS and we frequently call get_rflags() followed by a set_rflags() to update a bit. We also don't cache the value between calls and there's a few spots in the code that make multiple calls. Regards, Anthony Liguori