On Tue, Sep 13, 2022 at 01:49:30AM +0300, Kirill A. Shutemov wrote: > On Sun, Sep 04, 2022 at 03:39:52AM +0300, Kirill A. Shutemov wrote: > > On Thu, Sep 01, 2022 at 05:45:08PM +0000, Ashok Raj wrote: > > > Hi Kirill, > > > > > > On Tue, Aug 30, 2022 at 04:00:53AM +0300, Kirill A. Shutemov wrote: > > > > Linear Address Masking[1] (LAM) modifies the checking that is applied to > > > > 64-bit linear addresses, allowing software to use of the untranslated > > > > address bits for metadata. > > > > > > We discussed this internally, but didn't bubble up here. > > > > > > Given that we are working on enabling Shared Virtual Addressing (SVA) > > > within the IOMMU. This permits user to share VA directly with the device, > > > and the device can participate even in fixing page-faults and such. > > > > > > IOMMU enforces canonical addressing, since we are hijacking the top order > > > bits for meta-data, it will fail sanity check and we would return a failure > > > back to device on any page-faults from device. > > > > > > It also complicates how device TLB and ATS work, and needs some major > > > improvements to detect device capability to accept tagged pointers, adjust > > > the devtlb to act accordingly. > > > > > > > > > Both are orthogonal features, but there is an intersection of both > > > that are fundamentally incompatible. > > > > > > Its even more important, since an application might be using SVA under the > > > cover provided by some library that's used without their knowledge. > > > > > > The path would be: > > > > > > 1. Ensure both LAM and SVM are incompatible by design, without major > > > changes. > > > - If LAM is enabled already and later SVM enabling is requested by > > > user, that should fail. and Vice versa. > > > - Provide an API to user to ask for opt-out. Now they know they > > > must sanitize the pointers before sending to device, or the > > > working set is already isolated and needs no work. > > > > The patch below implements something like this. It is PoC, build-tested only. > > > > To be honest, I hate it. It is clearly a layering violation. It feels > > dirty. But I don't see any better way as we tie orthogonal features > > together. > > > > Also I have no idea how to make forced PASID allocation if LAM enabled. > > What the API has to look like? > > Jacob, Ashok, any comment on this part? > > I expect in many cases LAM will be enabled very early (like before malloc > is functinal) in process start and it makes PASID allocation always fail. > > Any way out? We need closure on this to proceed. Any clue? -- Kiryl Shutsemau / Kirill A. Shutemov
