From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37621) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZFzqB-00016X-5c for qemu-devel@nongnu.org; Fri, 17 Jul 2015 03:13:52 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZFzq7-0002Xw-4A for qemu-devel@nongnu.org; Fri, 17 Jul 2015 03:13:51 -0400 Received: from mail-pa0-f49.google.com ([209.85.220.49]:35985) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZFzq6-0002Xn-Rq for qemu-devel@nongnu.org; Fri, 17 Jul 2015 03:13:47 -0400 Received: by pachj5 with SMTP id hj5so56302350pac.3 for ; Fri, 17 Jul 2015 00:13:45 -0700 (PDT) References: <1436876514-2946-1-git-send-email-aik@ozlabs.ru> <1436876514-2946-5-git-send-email-aik@ozlabs.ru> <20150716051122.GA25179@voom.redhat.com> From: Alexey Kardashevskiy Message-ID: <55A8AB21.8080307@ozlabs.ru> Date: Fri, 17 Jul 2015 17:13:37 +1000 MIME-Version: 1.0 In-Reply-To: <20150716051122.GA25179@voom.redhat.com> Content-Type: text/plain; charset=koi8-r; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC PATCH qemu v3 4/4] vfio: spapr: Add SPAPR IOMMU v2 support (DMA memory preregistering) List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: Peter Crosthwaite , qemu-devel@nongnu.org, Michael Roth , Alex Williamson , qemu-ppc@nongnu.org, Paolo Bonzini On 07/16/2015 03:11 PM, David Gibson wrote: > On Tue, Jul 14, 2015 at 10:21:54PM +1000, Alexey Kardashevskiy wrote: >> This makes use of the new "memory registering" feature. The idea is >> to provide the userspace ability to notify the host kernel about pages >> which are going to be used for DMA. Having this information, the host >> kernel can pin them all once per user process, do locked pages >> accounting (once) and not spent time on doing that in real time with >> possible failures which cannot be handled nicely in some cases. >> >> This adds a guest RAM memory listener which notifies a VFIO container >> about memory which needs to be pinned/unpinned. VFIO MMIO regions >> (i.e. "skip dump" regions) are skipped. >> >> The feature is only enabled for SPAPR IOMMU v2. The host kernel changes >> are required. Since v2 does not need/support VFIO_IOMMU_ENABLE, this does >> not call it when v2 is detected and enabled. >> >> This does not change the guest visible interface. >> >> Signed-off-by: Alexey Kardashevskiy > > I've looked at this in more depth now, and attempting to unify the > pre-reg and mapping listeners like this can't work - they need to be > listening on different address spaces: mapping actions need to be > listening on the PCI address space, whereas the pre-reg needs to be > listening on address_space_memory. For x86 - for now - those end up > being the same thing, but on Power they're not. > > We do need to be clear about what differences are due to the presence > of a guest IOMMU versus which are due to arch or underlying IOMMU > type. For now Power has a guest IOMMU and x86 doesn't, but that could > well change in future: we could well implement the guest side IOMMU > for x86 in future (or x86 could invent a paravirt IOMMU interface). > On the other side, BenH's experimental powernv machine type could > introduce Power machines without a guest side IOMMU (or at least an > optional guest side IOMMU). > > The quick and dirty approach here is: > 1. Leave the main listener as is > 2. Add a new pre-reg notifier to the spapr iommu specific code, > which listens on address_space_memory, *not* the PCI space > > The more generally correct approach, which allows for more complex > IOMMU arrangements and the possibility of new IOMMU types with pre-reg > is: > 1. Have the core implement both a mapping listener and a pre-reg > listener (optionally enabled by a per-iommu-type flag). > Basically the first one sees what *is* mapped, the second sees > what *could* be mapped. > > 2. As now, the mapping listener listens on PCI address space, if > RAM blocks are added, immediately map them into the host IOMMU, > if guest IOMMU blocks appear register a notifier which will > mirror guest IOMMU mappings to the host IOMMU (this is what we > do now). > > 3. The pre-reg listener also listens on the PCI address space. RAM > blocks added are pre-registered immediately. PCI address space listeners won't be notified about RAM blocks on sPAPR. > But, if guest > IOMMU blocks are added, instead of registering a guest-iommu > notifier, "guest-iommu notifier" is the one called via memory_region_notify_iommu() from H_PUT_TCE? "Instead" implies dropping it, how this can work? > we register another listener on the *target* AS of the > guest IOMMU, same callbacks as this one. In practice that > target AS will almost always resolve to address_space_memory, > but this can at least in theory handle crazy guest setups with > multiple layers of IOMMU. > > 4. Have to ensure that the pre-reg callbacks always happen before > the mapping calls. For a system with an IOMMU backend which > requires pre-registration, but doesn't have a guest IOMMU, we > need to pre-reg, then host-iommu-map RAM blocks that appear in > PCI address space. -- Alexey