From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nadav Amit Subject: Re: [PATCH] iommu/vt-d: Remove unnecassary qi clflushes Date: Tue, 5 Jul 2016 09:50:39 -0700 Message-ID: <5F48A3E6-99EE-4DC5-BF38-4F0338E9E8D0@gmail.com> References: <1466773994-242872-1-git-send-email-namit@vmware.com> <20160627113014.GL11432@8bytes.org> <926A8CFB-D22D-4DC6-9EE0-D912451365BB@gmail.com> Mime-Version: 1.0 (Mac OS X Mail 9.3 \(3124\)) Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Paolo Bonzini Cc: Nadav Amit , iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: iommu@lists.linux-foundation.org Paolo Bonzini wrote: > > > On 05/07/2016 18:27, Nadav Amit wrote: >>> Although such hardware is old, there are some hypervisors that do not set >>> the ecap.coherency of emulated IOMMUs. Yes, it is unwise, but there is no >>> reason to further punish these hypervisors. > > QEMU will need the kernel to respect ecap.coherency in order to support > nested VFIO, for example. To clarify - the kernel respects the coherency, but performs more clflushes than necessary. It has no functional impact, but induces performance degradation (which I did not measure, but is likely to be several hundreds of cycles per flush). Regards, Nadav