From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: R/W HG memory mappings with kvm? Date: Thu, 27 Aug 2009 07:08:37 +0300 Message-ID: <4A9606C5.4060607@redhat.com> References: <5f370d430907051541o752d3dbag80d5cb251e5e4d00@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> <4A921D3C.6020809@redhat.com> <5f370d430908261934m15f39ab9mf54a19bdee1f278f@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]:4122 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751215AbZH0EIN (ORCPT ); Thu, 27 Aug 2009 00:08:13 -0400 In-Reply-To: <5f370d430908261934m15f39ab9mf54a19bdee1f278f@mail.gmail.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/27/2009 05:34 AM, Stephen Donnelly wrote: > On Mon, Aug 24, 2009 at 4:55 PM, Avi Kivity wrote: > >> 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. >> > If the vma->vm_file is /dev/mem, then the pg_off will map to physical > addresses directly (at least on x86), and the calculation works. If > the vma is remapping io memory from a driver, then vma->vm_file will > point to the device node for that driver. Perhaps we can do a check > for this at least? > We can't duplicate mm/ in kvm. However, mm/memory.c says: * The way we recognize COWed pages within VM_PFNMAP mappings is through the * rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit * set, and the vm_pgoff will point to the first PFN mapped: thus every special * mapping will always honor the rule * * pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT) * * And for normal mappings this is false. So it seems the kvm calculation is right and you should set vm_pgoff in your driver. > > >>> 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. >> > We can definitely map it into userspace. The problem seems to be how > the kvm kernel module translates the guest pfn back to a host physical > address. > > Is there a kernel equivalent of mmap? > do_mmap(), but don't use it. Use mmap() from userspace like everyone else. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.