From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id BC02246B9 for ; Thu, 14 Jul 2022 13:39:45 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A0D501CE2; Thu, 14 Jul 2022 06:39:39 -0700 (PDT) Received: from [10.57.86.49] (unknown [10.57.86.49]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A85EC3F73D; Thu, 14 Jul 2022 06:39:37 -0700 (PDT) Message-ID: Date: Thu, 14 Jul 2022 14:39:32 +0100 Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY Content-Language: en-GB To: Jean-Philippe Brucker Cc: joro@8bytes.org, will@kernel.org, virtualization@lists.linux-foundation.org, iommu@lists.linux.dev, eric.auger@redhat.com References: <20220714111059.708735-1-jean-philippe@linaro.org> From: Robin Murphy In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2022-07-14 14:00, Jean-Philippe Brucker wrote: > On Thu, Jul 14, 2022 at 01:01:37PM +0100, Robin Murphy wrote: >> On 2022-07-14 12:11, Jean-Philippe Brucker wrote: >>> Fix virtio-iommu interaction with VFIO, as VFIO now requires >>> IOMMU_CAP_CACHE_COHERENCY. virtio-iommu does not support non-cacheable >>> mappings, and always expects to be called with IOMMU_CACHE. >> >> Can we know this is actually true though? What if the virtio-iommu >> implementation is backed by something other than VFIO, and the underlying >> hardware isn't coherent? AFAICS the spec doesn't disallow that. > > Right, I should add a note about that. If someone does actually want to > support non-coherent device, I assume we'll add a per-device property, a > 'non-cacheable' mapping flag, and IOMMU_CAP_CACHE_COHERENCY will hold. > I'm also planning to add a check on (IOMMU_CACHE && !IOMMU_NOEXEC) in > viommu_map(), but not as a fix. But what about all the I/O-coherent PL330s? :P (IIRC you can actually make a Juno do that with S2CR.MTCFG hacks...) > In the meantime we do need to restore VFIO support under virtio-iommu, > since userspace still expects that to work, and the existing use-cases are > coherent devices. Yeah, I'm not necessarily against adding this as a horrible bodge for now - the reality is that people using VFIO must be doing it on coherent systems or it wouldn't be working properly anyway - as long as we all agree that that's what it is. Next cycle I'll be sending the follow-up patches to bring device_iommu_capable() to its final form (hoping the outstanding VDPA patch lands in the meantime), at which point we get to sort-of-fix the SMMU drivers[1], and can do something similar here too. I guess the main question for virtio-iommu is whether it needs to be described/negotiated in the protocol itself, or can be reliably described by other standard firmware properties (with maybe just a spec not to clarify that coherency must be consistent). Cheers, Robin. [1] https://gitlab.arm.com/linux-arm/linux-rm/-/commit/d8256bf48c8606cbaa6f0815696c2a6dbb72f1b0