From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: MTRR use in drivers Date: Sun, 23 Jun 2013 13:02:45 -0700 Message-ID: <51C75465.9060007@zytor.com> References: <51C3DDFA.7050204@zytor.com> <51C69732.1010906@gmail.com> <20130623192936.GA28655@khazad-dum.debian.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130623192936.GA28655@khazad-dum.debian.net> Sender: linux-kernel-owner@vger.kernel.org To: Henrique de Moraes Holschuh Cc: Brice Goglin , Linux Kernel Mailing List , David Airlie , dri-devel@lists.freedesktop.org List-Id: dri-devel@lists.freedesktop.org On 06/23/2013 12:29 PM, Henrique de Moraes Holschuh wrote: > On Sun, 23 Jun 2013, H. Peter Anvin wrote: >> Why do you care about performance when PAT is disabled? > > It will regress already slow boxes. We blacklist a LOT of P4s, PMs, etc and > nobody ever took the pain to track down which ones of those actually have > PAT+MTRR aliasing bugs. > > These boxes have boards like the Radeon X300, which needs either PAT or MTRR > to not become unusable... > We're talking hardware which is now many years old, but this is causing very serious problems on real, modern hardware. As far as I understand it, too, the blacklisting was precautionary (the only bug that I personally know about is a performance bug, where WC would be incorrectly converted to UC.) We need a way forward here. If it is the only way I think we would have to sacrifice the old machines, but perhaps something can be worked out (e.g. if PAT is disabled, fall back to MTRRs if available for ioremap_wc()). -hpa