Re: [PATCHv8 00/11] Linear Address Masking enabling

[Kirill A. Shutemov]

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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov