From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752894AbbIOQ3z (ORCPT ); Tue, 15 Sep 2015 12:29:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60255 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752336AbbIOQ3y (ORCPT ); Tue, 15 Sep 2015 12:29:54 -0400 Subject: Re: [PATCH V6 6/6] kvm: add fast mmio capabilitiy To: Cornelia Huck References: <1442299319-5253-1-git-send-email-jasowang@redhat.com> <1442299319-5253-7-git-send-email-jasowang@redhat.com> <55F8344B.50300@redhat.com> <20150915181319.097e341c.cornelia.huck@de.ibm.com> Cc: Jason Wang , gleb@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mst@redhat.com From: Paolo Bonzini Message-ID: <55F8477D.7040803@redhat.com> Date: Tue, 15 Sep 2015 18:29:49 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150915181319.097e341c.cornelia.huck@de.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/09/2015 18:13, Cornelia Huck wrote: > On Tue, 15 Sep 2015 17:07:55 +0200 > Paolo Bonzini wrote: > >> On 15/09/2015 08:41, Jason Wang wrote: >>> +With KVM_CAP_FAST_MMIO, a zero length mmio eventfd is allowed for >>> +kernel to ignore the length of guest write and get a possible faster >>> +response. Note the speedup may only work on some specific >>> +architectures and setups. Otherwise, it's as fast as wildcard mmio >>> +eventfd. >> >> I don't really like tying the capability to MMIO, especially since >> zero length ioeventfd is already accepted for virtio-ccw. > > Actually, zero length ioeventfd does not make sense for virtio-ccw; Can you explain why? If there is any non-zero valid length, "wildcard length" (represented by zero) would also make sense. Paolo > we just don't check it (although we probably should). > >> >> What about the following? >> >> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt >> index 7a3cb48a644d..247944071cc8 100644 >> --- a/Documentation/virtual/kvm/api.txt >> +++ b/Documentation/virtual/kvm/api.txt >> @@ -1627,11 +1627,10 @@ to the registered address is equal to datamatch in struct kvm_ioeventfd. >> For virtio-ccw devices, addr contains the subchannel id and datamatch the >> virtqueue index. >> >> -With KVM_CAP_FAST_MMIO, a zero length mmio eventfd is allowed for >> -kernel to ignore the length of guest write and get a possible faster >> -response. Note the speedup may only work on some specific >> -architectures and setups. Otherwise, it's as fast as wildcard mmio >> -eventfd. >> +With KVM_CAP_IOEVENTFD_ANY_LENGTH, a zero length ioeventfd is allowed, and >> +the kernel will ignore the length of guest write and get a faster vmexit. > > s/get/may get/ ? > >> +The speedup may only apply to specific architectures, but the ioeventfd will >> +work anyway. >