From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Williamson Subject: Re: VFIO iommu page size masking Date: Fri, 13 Feb 2015 10:31:32 -0600 Message-ID: <1423845092.5253.18.camel@redhat.com> References: <54DD6477.9050008@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: KVM , Varun Sethi , Will Deacon To: Alexander Graf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:37232 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752849AbbBMQbr (ORCPT ); Fri, 13 Feb 2015 11:31:47 -0500 In-Reply-To: <54DD6477.9050008@suse.de> Sender: kvm-owner@vger.kernel.org List-ID: On Fri, 2015-02-13 at 03:41 +0100, Alexander Graf wrote: > Hi Alex, > > While trying to get VFIO-PCI working on AArch64 (with 64k page size), I > stumbled over the following piece of code: > > > static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu) > > { > > struct vfio_domain *domain; > > unsigned long bitmap = PAGE_MASK; > > > > mutex_lock(&iommu->lock); > > list_for_each_entry(domain, &iommu->domain_list, next) > > bitmap &= domain->domain->ops->pgsize_bitmap; > > mutex_unlock(&iommu->lock); > > > > return bitmap; > > } > > The SMMU page mask is > > [ 3.054302] arm-smmu e0a00000.smmu: Supported page sizes: 0x40201000 > > but after this function, we end up supporting one 2MB pages and above. > The reason for that is simple: You restrict the bitmap to PAGE_MASK and > above. > > Now the big question is why you're doing that. I don't see why it would > be a problem if the IOMMU maps a page in smaller chunks. > > So I tried to patch the code above with s/PAGE_MASK/1UL/ and everything > seems to run fine. But maybe we're not lacking some sanity checks? Hey Alex, Yeah, we may need to double check if we prevent sub-PAGE_SIZE mappings elsewhere in the DMA mapping path, but that's probably the right thing to do. On x86 we have AMD-Vi, which actually supports just about any power-of-two mapping and therefore exposes effectively PAGE_MASK and VT-d, which only natively supports a few page sizes, but breaks down mappings itself and therefore muddies the interface by exposing PAGE_MASK also. So the IOMMU API ends up not really being a way to expose native IOMMU page sizes anyway. BTW, I'm on holiday until late next week, so I apologize to all the vfio threads that won't be getting any attention until then. Thanks, Alex