Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752283AbbBRE1q (ORCPT ); Tue, 17 Feb 2015 23:27:46 -0500 Received: from mx1.redhat.com ([209.132.183.28]:53318 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751576AbbBRE1o (ORCPT ); Tue, 17 Feb 2015 23:27:44 -0500 Date: Wed, 18 Feb 2015 05:27:39 +0100 From: "Michael S. Tsirkin" To: Eric Northup Cc: Paolo Bonzini , Igor Mammedov , Linux Kernel Mailing List , KVM , netdev@vger.kernel.org Subject: Re: [PATCH] vhost: support upto 509 memory regions Message-ID: <20150218042739.GA22891@redhat.com> References: <1423842599-5174-1-git-send-email-imammedo@redhat.com> <20150217090242.GA20254@redhat.com> <54E31F24.1060705@redhat.com> <20150217123212.GA6362@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2082 Lines: 61 On Tue, Feb 17, 2015 at 04:53:45PM -0800, Eric Northup wrote: > On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin wrote: > > On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: > >> > >> > >> On 17/02/2015 10:02, Michael S. Tsirkin wrote: > >> > > Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 > >> > > to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. > >> > > > >> > > Signed-off-by: Igor Mammedov > >> > > >> > This scares me a bit: each region is 32byte, we are talking > >> > a 16K allocation that userspace can trigger. > >> > >> What's bad with a 16K allocation? > > > > It fails when memory is fragmented. > > > >> > How does kvm handle this issue? > >> > >> It doesn't. > >> > >> Paolo > > > > I'm guessing kvm doesn't do memory scans on data path, > > vhost does. > > > > qemu is just doing things that kernel didn't expect it to need. > > > > Instead, I suggest reducing number of GPA<->HVA mappings: > > > > you have GPA 1,5,7 > > map them at HVA 11,15,17 > > then you can have 1 slot: 1->11 > > > > To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE > > or something like this. > > This works beautifully when host virtual address bits are more > plentiful than guest physical address bits. Not all architectures > have that property, though. AFAIK this is pretty much a requirement for both kvm and vhost, as we require each guest page to also be mapped in qemu memory. > > We can discuss smarter lookup algorithms but I'd rather > > userspace didn't do things that we then have to > > work around in kernel. > > > > > > -- > > MST > > -- > > To unsubscribe from this list: send the line "unsubscribe kvm" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/