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

[Jerome Glisse]

On Thu, Mar 21, 2019 at 07:51:16PM +0000, Thomas Hellstrom wrote:
> 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.

You already provide the mapping as argument it should not be hard to
check it is a mapping to a device file as the vma flags will not be
enough to identify this case.

> 
> > 
> > > 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?

Both i believe, however i have not try to apply your patches on top of
the userfaultfd patchset

Cheers,
Jérôme