Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

[Jason Gunthorpe]

On Fri, Apr 30, 2021 at 10:54:17AM +0100, Lorenzo Pieralisi wrote:
> [+Jason, Ben]
> 
> On Thu, Apr 29, 2021 at 11:29:05AM -0500, Shanker Donthineni wrote:
> > For pass-through device assignment, the ARM64 KVM hypervisor retrieves
> > the memory region properties physical address, size, and whether a
> > region backed with struct page or not from VMA. The prefetchable
> > attribute of a BAR region isn't visible to KVM to make an optimal
> > decision for stage2 attributes.
> > 
> > This patch updates vma->vm_page_prot and maps with write-combine
> > attribute if the associated BAR is prefetchable. For ARM64
> > pgprot_writecombine() is mapped to memory-type MT_NORMAL_NC which
> > has no side effects on reads and multiple writes can be combined.
> > 
> > Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
> >  drivers/vfio/pci/vfio_pci.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> A bit of background information that may be useful:
> 
> https://lore.kernel.org/linux-pci/2b539df4c9ec703458e46da2fc879ee3b310b31c.camel@xxxxxxxxxxxxxxxxxxx

This can't happen automatically.

writecombining or not is a uABI visible change. Userspace needs to
explicitly request and know that the mmap it gets back is
writecombining.

Jason