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

[Ashok Raj]

On Wed, Sep 21, 2022 at 10:11:46AM -0700, Dave Hansen wrote:
> On 9/21/22 10:08, Ashok Raj wrote:
> > On Wed, Sep 21, 2022 at 09:57:47AM -0700, Dave Hansen wrote:
> >> On 9/15/22 10:28, Kirill A. Shutemov wrote:> +	/* Serialize against
> >> address tagging enabling *
> >>> +	if (mmap_write_lock_killable(mm))
> >>> +		return -EINTR;
> >>> +
> >>> +	if (!arch_can_alloc_pasid(mm)) {
> >>> +		mmap_write_unlock(mm);
> >>> +		return -EBUSY;
> >>> +	}
> >> Shouldn't this actually be some kind of *device* check?
> > The device will enable svm only when its capable of it, and performs all
> > the normal capability checks like PASID, ATS etc before enabling it.
> > This is the final step before the mm is hooked up with the IOMMU.
> 
> What does that mean, though?
> 
> Are you saying that any device compatibility with an mm is solely
> determined by the IOMMU in play, so the IOMMU code should host the mm
> compatibility checks?
> 

To check if a device supports SVM like capabilities it needs to support
the following PCIe capabilities.

- PASID
- Page Request Interface (PRI) for dynamic page-faulting
- ATS - For quick VA->PA lookups.

The device purely works only with memory addresses and caches them in
its device TLB after a lookup via ATS. 

When device does ATS, it sends a translation request, and IOMMU will
walk the page-tables to give the PA back. It can use it until it gets an
invalidation. 

So the device doesn't need to know page-table formats. but if you use
tagged pointers its something you want to check device support for it. I
don't think there is any plans right now to support something like the
following.

- Check device ability to work with tagged pointers.
- OS should configure the width to ignore etc
- Device TLB's properly handle the tagged portion without creating
  aliasing etc.

In order for LAM and SVM to play nicely you need 

#1 IOMMU support for tagged pointers 
#2 Device ability to handle tagged pointers. 

#2 above is an additional check to perform in addition to PASID,PRI,ATS
   checks we do today.

Cheers,
Ashok