From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757487Ab0JVQoU (ORCPT ); Fri, 22 Oct 2010 12:44:20 -0400 Received: from terminus.zytor.com ([198.137.202.10]:56298 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754548Ab0JVQoT (ORCPT ); Fri, 22 Oct 2010 12:44:19 -0400 Message-ID: <4CC1BF58.9020001@zytor.com> Date: Fri, 22 Oct 2010 09:44:08 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.9) Gecko/20100921 Fedora/3.1.4-1.fc13 Thunderbird/3.1.4 MIME-Version: 1.0 To: Konrad Rzeszutek Wilk CC: Jeremy Fitzhardinge , the arch/x86 maintainers , "Xen-devel@lists.xensource.com" , Linux Kernel Mailing List Subject: Re: [PATCH] x86: define arch_vm_get_page_prot to set _PAGE_IOMAP on VM_IO vmas References: <4CC0C14E.5080205@goop.org> <4CC0C318.90401@zytor.com> <4CC0CA07.3000306@goop.org> <4CC0DEB8.1060309@zytor.com> <20101022150826.GA23325@dumpdata.com> In-Reply-To: <20101022150826.GA23325@dumpdata.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/22/2010 08:08 AM, Konrad Rzeszutek Wilk wrote: >> >> Okay, could you clarify this part a bit? Why does the kernel need to >> know the difference between "pseudo-physical" and "machine addresses" at >> all? If they overlap, there is a problem, and if they don't overlap, it >> will be a 1:1 mapping anyway... > > The flag (_PAGE_IOMAP) is used when we set the PTE so that the MFN value is > used instead of the PFN. We need that b/c when a driver does page_to_pfn() > it ends up using the PFN as bus address to write out registers data. > > Without this patch, the page->virt->PFN value is used and the PFN != to real MFN > so we end up writing in a memory address that the PCI device has no idea about. > By setting the PTE with the MFN, the virt->PFN gets the real MFN value. > > The drivers I am talking about are mostly, if not all, located in drivers/gpu > and it looks that we are missing two more patches to utilize the patch > that Jeremy posted. > > Please note that I am _not_ suggesting that the two patches > below should go out - I still need to post them on drm mailing list. > I'm still seriously confused. If I understand this correctly, we're talking about DMA addresses here (as opposed to PIO addresses, i.e. BARs), right? It's the bimodality that really bothers me. I understand of course that Xen imposes yet another address remapping layer, but I'm having a hard time understanding any conditions under with we would need that layer to go away, as long as DMA addresses are translated via the DMA APIs -- and if they aren't, then iommus will break, too. As such, I don't grok this page flag and what it does, and why it's needed. I'm not saying it's *wrong*, I'm saying the design is opaque to me and I'm not sure it is the right solution. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.