Re: [PATCH 00/19] mm: Support huge pfnmaps

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 14, 2024, Jason Gunthorpe wrote:
> On Fri, Aug 09, 2024 at 12:08:50PM -0400, Peter Xu wrote:
> > Overview
> > ========
> > 
> > This series is based on mm-unstable, commit 98808d08fc0f of Aug 7th latest,
> > plus dax 1g fix [1].  Note that this series should also apply if without
> > the dax 1g fix series, but when without it, mprotect() will trigger similar
> > errors otherwise on PUD mappings.
> > 
> > This series implements huge pfnmaps support for mm in general.  Huge pfnmap
> > allows e.g. VM_PFNMAP vmas to map in either PMD or PUD levels, similar to
> > what we do with dax / thp / hugetlb so far to benefit from TLB hits.  Now
> > we extend that idea to PFN mappings, e.g. PCI MMIO bars where it can grow
> > as large as 8GB or even bigger.
> 
> FWIW, I've started to hear people talk about needing this in the VFIO
> context with VMs.
> 
> vfio/iommufd will reassemble the contiguous range from the 4k PFNs to
> setup the IOMMU, but KVM is not able to do it so reliably.

Heh, KVM should very reliably do the exact opposite, i.e. KVM should never create
a huge page unless the mapping is huge in the primary MMU.  And that's very much
by design, as KVM has no knowledge of what actually resides at a given PFN, and
thus can't determine whether or not its safe to create a huge page if KVM happens
to realize the VM has access to a contiguous range of memory.