From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752208Ab2HMSV6 (ORCPT ); Mon, 13 Aug 2012 14:21:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13428 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751377Ab2HMSV5 (ORCPT ); Mon, 13 Aug 2012 14:21:57 -0400 Date: Mon, 13 Aug 2012 14:39:00 -0300 From: Marcelo Tosatti To: Xiao Guangrong Cc: Avi Kivity , LKML , KVM Subject: Re: [PATCH v5 00/12] KVM: introduce readonly memslot Message-ID: <20120813173900.GA25268@amt.cnet> References: <5020E423.9080004@linux.vnet.ibm.com> <20120810181422.GA14892@amt.cnet> <5025D334.9070503@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5025D334.9070503@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 11, 2012 at 11:36:20AM +0800, Xiao Guangrong wrote: > On 08/11/2012 02:14 AM, Marcelo Tosatti wrote: > > On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote: > >> Changelog: > >> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page > >> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators > >> > >> The test case can be found at: > >> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2 > >> > >> In current code, if we map a readonly memory space from host to guest > >> and the page is not currently mapped in the host, we will get a fault-pfn > >> and async is not allowed, then the vm will crash. > >> > >> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD > >> to the guest, read access is happy for readonly memslot, write access on > >> readonly memslot will cause KVM_EXIT_MMIO exit. > > > > Memory slots whose QEMU mapping is write protected is supported > > today, as long as there are no write faults. > > > > What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots > > again? > > > > It is happy to map !write host memory space to the readonly memslot, > and they can coexist as well. > > readonly memslot checks the write-permission by seeing slot->flags and > !write memory checks the write-permission in hva_to_pfn() function > which checks vma->flags. It is no conflict. Yes, there is no conflict. The point is, if you can use the mmap(PROT_READ) interface (supporting read faults on read-only slots) for this behavior, what is the advantage of a new memslot flag? I'm not saying mmap(PROT_READ) is the best interface, i am just asking why it is not. > > The initial objective was to fix a vm crash, can you explain that > > initial problem? > > > > The issue was trigged by this code: > > } else { > if (async && (vma->vm_flags & VM_WRITE)) > *async = true; > pfn = KVM_PFN_ERR_FAULT; > } > > If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and > its physical page is swapped out (or the file data does not be read in), > get_user_page_nowait will fail, above code reject to set async, > then we will get a fault pfn and async=false. > > I guess this issue also exists in "QEMU write protected mapping" as > you mentioned above. Yes, it does. As far as i understand, what that check does from a high level pov is: - Did get_user_pages_nowait() fail due to a swapped out page (in which case we should try to swappin the page asynchronously), or due to another reason (for which case an error should be returned). Using vma->vm_flags VM_WRITE for that is trying to guess why get_user_pages_nowait() failed, because it (gup_nowait return values) does not provide sufficient information by itself. Can't that be fixed separately? Another issue which is also present with the mmap(PROT_READ) scheme is interaction with reexecute_instruction. That is, unless i am mistaken, reexecute_instruction can succeed (return true) on a region that is write protected. This breaks the "write faults on read-only slots exit to userspace via EXIT_MMIO" behaviour.