From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38649) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YvTfn-0002Y4-W9 for qemu-devel@nongnu.org; Thu, 21 May 2015 12:50:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YvTfj-0006Rd-O6 for qemu-devel@nongnu.org; Thu, 21 May 2015 12:50:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43527) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YvTfj-0006RJ-HV for qemu-devel@nongnu.org; Thu, 21 May 2015 12:50:15 -0400 Date: Thu, 21 May 2015 18:50:02 +0200 From: Andrew Jones Message-ID: <20150521165002.GD2898@localhost.localdomain> References: <1431516714-25816-1-git-send-email-drjones@redhat.com> <1431516714-25816-4-git-send-email-drjones@redhat.com> <20150514105549.GO32765@cbox> <20150514133213.GD12812@localhost.localdomain> <20150515145921.GB14144@lvm> <20150515170437.GA2951@localhost.localdomain> <555D4308.3030504@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <555D4308.3030504@samsung.com> Subject: Re: [Qemu-devel] [RFC/RFT PATCH v2 3/3] arm/arm64: KVM: implement 'uncached' mem coherency List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Mario Smarduch Cc: peter.maydell@linaro.org, ard.biesheuvel@linaro.org, marc.zyngier@arm.com, catalin.marinas@arm.com, qemu-devel@nongnu.org, agraf@suse.de, pbonzini@redhat.com, j.fanguede@virtualopensystems.com, lersek@redhat.com, kvmarm@lists.cs.columbia.edu, Christoffer Dall On Wed, May 20, 2015 at 07:29:28PM -0700, Mario Smarduch wrote: > On 05/15/2015 10:04 AM, Andrew Jones wrote: > > On Fri, May 15, 2015 at 08:02:59AM -0700, Christoffer Dall wrote: > >> On Thu, May 14, 2015 at 03:32:13PM +0200, Andrew Jones wrote: > >>> On Thu, May 14, 2015 at 12:55:49PM +0200, Christoffer Dall wrote: > >>>> On Wed, May 13, 2015 at 01:31:54PM +0200, Andrew Jones wrote: > >>>>> When S1 and S2 memory attributes combine wrt to caching policy, > >>>>> non-cacheable types take precedence. If a guest maps a region as > >>>>> device memory, which KVM userspace is using to emulate the device > >>>>> using normal, cacheable memory, then we lose coherency. With > >>>>> KVM_MEM_UNCACHED, KVM userspace can now hint to KVM which memory > >>>>> regions are likely to be problematic. With this patch, as pages > >>>>> of these types of regions are faulted into the guest, not only do > >>>>> we flush the page's dcache, but we also change userspace's > >>>>> mapping to NC in order to maintain coherency. > >>>>> > >>>>> What if the guest doesn't do what we expect? While we can't > >>>>> force a guest to use cacheable memory, we can take advantage of > >>>>> the non-cacheable precedence, and force it to use non-cacheable. > >>>>> So, this patch also introduces PAGE_S2_NORMAL_NC, and uses it on > >>>>> KVM_MEM_UNCACHED regions to force them to NC. > >>>>> > >>>>> We now have both guest and userspace on the same page (pun intended) > >>>> > >>>> I'd like to revisit the overall approach here. Is doing non-cached > >>>> accesses in both the guest and host really the right thing to do here? > >>> > >>> I think so, but all ideas/approaches are still on the table. This is > >>> still an RFC. > >>> > >>>> > >>>> The semantics of the device becomes that it is cache coherent (because > >>>> QEMU is), and I think Marc argued that Linux/UEFI should simply be > >>>> adapted to handle whatever emulated devices we have as coherent. I also > >>>> remember someone arguing that would be wrong (Peter?). > >>> > >>> I'm not really for quirking all devices in all guest types (AAVMF, Linux, > >>> other bootloaders, other OSes). Windows is unlikely to apply any quirks. > >>> > >> > >> Well my point was that if we're emulating a platform with coherent IO > >> memory for PCI devices that is something that the guest should work with > >> as such, but as Paolo explained it should always be safe for a guest to > >> assume non-coherent, so that doesn't work. > >> > >>>> > >>>> Finally, does this address all cache coherency issues with emulated > >>>> devices? Some VOS guys had seen things still not working with this > >>>> approach, unsure why... I'd like to avoid us merging this only to merge > >>>> a more complete solution in a few weeks which reverts this solution... > >>> > >>> I'm not sure (this is still an RFT too :-) We definitely would need to > >>> scatter some more memory_region_set_uncached() calls around QEMU first. > >>> > >> > >> It would be good if you could sync with the VOS guys and make sure your > >> patch set addresses their issues with the appropriate > >> memory_region_set_uncached() added to QEMU, and if it does not, some > >> vague idea why that falls outside of the scope of this patch set. After > >> all, adding a USB controller to a VM is not that an esoteric use case, > >> is it? > > > > I'll pull together a new version addressing all your comments, and also > > put some more time into making sure it'll work... > > > > Jeremy, can you give me the qemu command line you're using for your tests? > > I'll do some experimenting with it. > > > > Thanks, > > drew > > > > Hi Drew, > > I just recently looked at these patches and little confused. > > Where or how are the QEMU page tables changed to > non-cacheable? > > I noticed the logical pfn is changed to non-cacheable. Right, this series is broken. Please stay tuned, I'll fix it the next time around. Thanks for reviewing. drew