From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v2] KVM: SVM: Sync g_pat with guest-written PAT value Date: Tue, 21 Apr 2015 14:12:44 +0200 Message-ID: <55363EBC.8000202@redhat.com> References: <552B5128.4010909@siemens.com> <552B6923.3020602@siemens.com> <20150420161401.GB26491@potion.brq.redhat.com> <5535368B.9060408@siemens.com> <55362FFC.1040905@redhat.com> <553633BA.7020900@siemens.com> <5536354F.5000503@redhat.com> <55363B04.20108@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , kvm , Joel Schopp To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:46824 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751473AbbDUMMt (ORCPT ); Tue, 21 Apr 2015 08:12:49 -0400 In-Reply-To: <55363B04.20108@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On 21/04/2015 13:56, Jan Kiszka wrote: > > Basically it's an optimization. The guest can set the UC memory type on > > PCI BARs that are actually backed by RAM in QEMU, and then accesses to > > these BARs will be unnecessarily slow. It would be particularly bad if, > > for example, access to ivshmem were slowed down because the guest PAT > > says the memory is uncacheable. > > ivshmem is pv anyway - why shouldn't the guest driver take this room for > optimization into account and ask for a cached mapping? > > Is that that only use case? I guess a frame buffer would be affected as well, though probably the guest would set it to WC so it's less bad. Paolo