Re: [RFC PATCH RESEND 0/3] mm modifications / helpers for emulated GPU coherent memory

[Thomas Hellstrom <thellstrom@vmware.com>]

Hi, Jérôme,

Thanks for commenting. I have a couple of questions / clarifications
below.

On Thu, 2019-03-21 at 09:46 -0400, Jerome Glisse wrote:
> On Thu, Mar 21, 2019 at 01:22:22PM +0000, Thomas Hellstrom wrote:
> > Resending since last series was sent through a mis-configured SMTP
> > server.
> > 
> > Hi,
> > This is an early RFC to make sure I don't go too far in the wrong
> > direction.
> > 
> > Non-coherent GPUs that can't directly see contents in CPU-visible
> > memory,
> > like VMWare's SVGA device, run into trouble when trying to
> > implement
> > coherent memory requirements of modern graphics APIs. Examples are
> > Vulkan and OpenGL 4.4's ARB_buffer_storage.
> > 
> > To remedy, we need to emulate coherent memory. Typically when it's
> > detected
> > that a buffer object is about to be accessed by the GPU, we need to
> > gather the ranges that have been dirtied by the CPU since the last
> > operation,
> > apply an operation to make the content visible to the GPU and clear
> > the
> > the dirty tracking.
> > 
> > Depending on the size of the buffer object and the access pattern
> > there are
> > two major possibilities:
> > 
> > 1) Use page_mkwrite() and pfn_mkwrite(). (GPU buffer objects are
> > backed
> > either by PCI device memory or by driver-alloced pages).
> > The dirty-tracking needs to be reset by write-protecting the
> > affected ptes
> > and flush tlb. This has a complexity of O(num_dirty_pages), but the
> > write page-fault is of course costly.
> > 
> > 2) Use hardware dirty-flags in the ptes. The dirty-tracking needs
> > to be reset
> > by clearing the dirty bits and flush tlb. This has a complexity of
> > O(num_buffer_object_pages) and dirty bits need to be scanned in
> > full before
> > each gpu-access.
> > 
> > So in practice the two methods need to be interleaved for best
> > performance.
> > 
> > So to facilitate this, I propose two new helpers,
> > apply_as_wrprotect() and
> > apply_as_clean() ("as" stands for address-space) both inspired by
> > unmap_mapping_range(). Users of these helpers are in the making,
> > but needs
> > some cleaning-up.
> 
> To be clear this should _only be use_ for mmap of device file ? If so
> the API should try to enforce that as much as possible for instance
> by
> mandating the file as argument so that the function can check it is
> only use in that case. Also big scary comment to make sure no one
> just
> start using those outside this very limited frame.

Fine with me. Perhaps we could BUG() / WARN() on certain VMA flags 
instead of mandating the file as argument. That can make sure we
don't accidently hit pages we shouldn't hit.

> 
> > There's also a change to x_mkwrite() to allow dropping the mmap_sem
> > while
> > waiting.
> 
> This will most likely conflict with userfaultfd write protection. 

Are you referring to the x_mkwrite() usage itself or the mmap_sem
dropping facilitation?

> Maybe
> building your thing on top of that would be better.
> 
> 
...
> 
> I will take a cursory look at the patches.
> 

Some more questions / clarifications on those as well.


> Cheers,
> Jérôme