From mboxrd@z Thu Jan 1 00:00:00 1970 From: Malcolm Crossley Subject: Re: [PATCH] iommu/quirk: disable shared EPT for Sandybridge and earlier processors. Date: Thu, 26 Nov 2015 13:48:33 +0000 Message-ID: <56570DB1.7020800@citrix.com> References: <1448385479-17614-1-git-send-email-anshul.makkar@citrix.com> <5654AF4C02000078000B8A11@prv-mh.provo.novell.com> <56558D35.2040800@citrix.com> <56571B3202000078000B9652@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <56571B3202000078000B9652@prv-mh.provo.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich , Andrew Cooper Cc: yang.z.zhang@intel.com, Anshul Makkar , kevin.tian@intel.com, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On 26/11/15 13:46, Jan Beulich wrote: >>>> On 25.11.15 at 11:28, wrote: >> The problem is that SandyBridge IOMMUs advertise 2M support and do >> function with it, but cannot cache 2MB translations in the IOTLBs. >> >> As a result, attempting to use 2M translations causes substantially >> worse performance than 4K translations. > > Btw - how does this get explained? At a first glance, even if 2Mb > translations don't get entered into the TLB, it should still be one > less page table level to walk for the IOMMU, and should hence > nevertheless be a benefit. Yet you even say _substantially_ > worse performance results. There is a IOTLB for the 4K translation so if you only use 4K translations then you get to take advantage of the IOTLB. If you use the 2Mb translation then a page table walk has to be performed every time there's a DMA access to that region of the BFN address space. Malcolm > > Jan >