From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH v2] KVM: SVM: Sync g_pat with guest-written PAT value Date: Tue, 21 Apr 2015 13:56:52 +0200 Message-ID: <55363B04.20108@siemens.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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , kvm , Joel Schopp To: Paolo Bonzini Return-path: Received: from thoth.sbs.de ([192.35.17.2]:36818 "EHLO thoth.sbs.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753568AbbDUL5A (ORCPT ); Tue, 21 Apr 2015 07:57:00 -0400 In-Reply-To: <5536354F.5000503@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2015-04-21 13:32, Paolo Bonzini 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? Jan -- Siemens AG, Corporate Technology, CT RTC ITP SES-DE Corporate Competence Center Embedded Linux