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

[Kirill A. Shutemov]

On Mon, Sep 12, 2022 at 05:08:09PM -0700, Jacob Pan wrote:
> Hi Kirill,
> 
> On Tue, 13 Sep 2022 01:49:30 +0300, "Kirill A. Shutemov"
> <kirill.shutemov@xxxxxxxxxxxxxxx> 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.
> > 
> Is there a generic flag LAM can set on the mm?

Hm. Not really.

I thought we can use untagged_addr(mm, -1UL) != -1UL as such check, but
-1UL is kernel address and untagged_addr() would not untag such address
for LAM.

I guess we can make add a helper for this.

But tagged address implementation is rather different across different
platforms and semantic can be hard to define. Like if the tagged addresses
support per-thread or per-process. Or maybe it is global.

Maybe just add arch hook there? arch_can_alloc_pasid(mm) or something.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov