From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: [PATCH] VT-d: add iommu=igfx_off option to workaround graphics issues Date: Tue, 21 Jul 2015 00:56:57 -0600 Message-ID: <55AE0959020000780009379E@prv-mh.provo.novell.com> References: <1437159952-4134-1-git-send-email-lantw44@gmail.com> <55ACAF80.4000608@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Kevin Tian Cc: Ting-Wei Lan , Andrew Cooper , Yang Z Zhang , "xen-devel@lists.xen.org" List-Id: xen-devel@lists.xenproject.org >>> On 21.07.15 at 02:57, wrote: >> From: Andrew Cooper [mailto:amc96@hermes.cam.ac.uk] On Behalf Of Andrew > Cooper >> Sent: Monday, July 20, 2015 4:21 PM >> >> On 20/07/2015 02:28, Tian, Kevin wrote: >> >> From: Ting-Wei Lan [mailto:lantw44@gmail.com] >> >> Sent: Saturday, July 18, 2015 3:06 AM >> >> >> >> When using Linux >= 3.19 (commit 47591df) as dom0 on some Intel Ironlake >> >> devices, It is possible to encounter graphics issues that make screen >> >> unreadable or crash the system. It was reported in freedesktop bugzilla: >> >> >> >> https://bugs.freedesktop.org/show_bug.cgi?id=90037 >> >> >> >> As we still cannot find a proper fix for this problem, this patch adds >> >> iommu=igfx_off option that is similar to Linux intel_iommu=igfx_off for >> >> users to manually workaround the problem. >> >> >> >> Signed-off-by: Ting-Wei Lan >> > Since igfx works before, I'd think a more proper fix should be on the >> > bisected Linux commit or i915 to have two working correctly together. >> > Otherwise this patch is just hiding problem. >> >> The linux commit is the one which actually fixes PAT support for Linux >> under Xen. >> >> It will cause the i915 driver to actually get WC mappings when it asks >> for them. > > This is the part which I don't quite understand. WC is essentially an UC > attribute with write buffer to accelerate the write efficiency. There > should be no correctness problem to use either WC or UC if i915 driver > wants WC. "Should" is too weak a term here: Using WC on the wrong piece of memory or without the necessary fencing can imo very well cause correctness problems (which would be hidden by WC -> UC conversion behind the driver's back). Jan