From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 4/4] Reallocate dma buffers in read/write path if needed Date: Sat, 04 Oct 2008 13:00:45 +0300 Message-ID: <48E73ECD.9080309@redhat.com> References: <1223071531-31817-1-git-send-email-ryanh@us.ibm.com> <1223071531-31817-5-git-send-email-ryanh@us.ibm.com> <200810040017.09081.paul@codesourcery.com> <48E6AC36.3060404@codemonkey.ws> Reply-To: qemu-devel@nongnu.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Anthony Liguori , Ryan Harper , Paul Brook , kvm@vger.kernel.org To: qemu-devel@nongnu.org Return-path: In-Reply-To: <48E6AC36.3060404@codemonkey.ws> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org List-Id: kvm.vger.kernel.org Anthony Liguori wrote: > Paul Brook wrote: >> On Friday 03 October 2008, Ryan Harper wrote: >> >>> The default buffer size breaks up larger read/write requests >>> unnecessarily. >>> When we encounter requests larger than the default dma buffer, >>> reallocate >>> the buffer to support the request. >>> >> >> Allocating unboundedly large host buffers based on guest input seems >> like a bad idea. >> > > Perhaps they could be at least bound to phys_ram_size. > So the guest could double the memory load on the host with just one request?! > In general, I don't think there's a correct size to bound them that's > less than phys_ram_size. The guest may be issuing really big IO > requests. The correct fix is not to buffer at all but use scatter-gather. Until this is done buffering has to be bounded. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.