From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: R/W HG memory mappings with kvm? Date: Mon, 24 Aug 2009 07:55:24 +0300 Message-ID: <4A921D3C.6020809@redhat.com> References: <5f370d430907051541o752d3dbag80d5cb251e5e4d00@mail.gmail.com> <5f370d430907081501m60064c7dp23ebd4153c9050f1@mail.gmail.com> <5f370d430907262256rd7f9fdalfbbec1f9492ce86@mail.gmail.com> <4A6DBE54.3080609@cs.ualberta.ca> <5f370d430907271432y5283c2cat7673efeed0febe20@mail.gmail.com> <4A6EBCB3.4080804@redhat.com> <5f370d430907281606j77f0c1a6j5feb081daca187ff@mail.gmail.com> <5f370d430908122107j15acd2c7i96d476e69032fadd@mail.gmail.com> <4A8BEC92.6070105@redhat.com> <5f370d430908231459q4c8cfe3j62c49e33a160ab71@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Cam Macdonell , "kvm@vger.kernel.org list" To: Stephen Donnelly Return-path: Received: from mx1.redhat.com ([209.132.183.28]:23281 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751451AbZHXEy6 (ORCPT ); Mon, 24 Aug 2009 00:54:58 -0400 In-Reply-To: <5f370d430908231459q4c8cfe3j62c49e33a160ab71@mail.gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/24/2009 12:59 AM, Stephen Donnelly wrote: > On Thu, Aug 20, 2009 at 12:14 AM, Avi Kivity wrote: > >> On 08/13/2009 07:07 AM, Stephen Donnelly wrote: >> >>> npages = get_user_pages_fast(addr, 1, 1, page); returns -EFAULT, >>> presumably because (vma->vm_flags& (VM_IO | VM_PFNMAP)). >>> >>> It takes then unlikely branch, and checks the vma, but I don't >>> understand what it is doing here: pfn = ((addr - vma->vm_start)>> >>> PAGE_SHIFT) + vma->vm_pgoff; >>> >> It's calculating the pfn according to pfnmap rules. >> > From what I understand this will only work when remapping 'main > memory', e.g. where the pgoff is equal to the physical page offset? > VMAs that remap IO memory will usually set pgoff to 0 for the start of > the mapping. > If so, how do they calculate the pfn when mapping pages? kvm needs to be able to do the same thing. >>> In my case addr == vma->vm_start, and vma->vm_pgoff == 0, so pfn ==0. >>> >> How did you set up that vma? It should point to the first pfn of your >> special memory area. >> > The vma was created with a remap_pfn_range call from another driver. > Because this call sets VM_PFNMAP and VM_IO any get_user_pages(_fast) > calls will fail. > > In this case the host driver was actually just remapping host memory, > so I replaced the remap_pfn_range call with a nopage/fault vm_op. This > allows the get_user_pages_fast call to succeed, and the mapping now > works as expected. This is sufficient for my work at the moment. > > Well if the fix is correct we need it too. > I'm still not sure how genuine IO memory (mapped from a driver to > userspace with remap_pfn_range or io_remap_page_range) could be mapped > into kvm though. > If it can be mapped to userspace, it can be mapped to kvm. We just need to synchronize the rules. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.