linux-kernel - Re: [RFC PATCH v1 05/37] KVM: guest_memfd: Wire up kvm_get_memory

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Message-ID: <20260129003753.GZ1641016@ziepe.ca>
Date: Wed, 28 Jan 2026 20:37:53 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: Alexey Kardashevskiy <aik@....com>, cgroups@...r.kernel.org,
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Subject: Re: [RFC PATCH v1 05/37] KVM: guest_memfd: Wire up
 kvm_get_memory_attributes() to per-gmem attributes

On Wed, Jan 28, 2026 at 01:47:50PM -0800, Ackerley Tng wrote:
> Alexey Kardashevskiy <aik@....com> writes:
> 
> >
> > [...snip...]
> >
> >
> 
> Thanks for bringing this up!
> 
> > I am trying to make it work with TEE-IO where fd of VFIO MMIO is a dmabuf fd while the rest (guest RAM) is gmemfd. The above suggests that if there is gmemfd - then the memory attributes are handled by gmemfd which is... expected?
> >
> 
> I think this is not expected.
> 
> IIUC MMIO guest physical addresses don't have an associated memslot, but
> if you managed to get to that line in kvm_gmem_get_memory_attributes(),
> then there is an associated memslot (slot != NULL)?

I think they should have a memslot, shouldn't they? I imagine creating
a memslot from a FD and the FD can be memfd, guestmemfd, dmabuf, etc,
etc ?

> Either way, guest_memfd shouldn't store attributes for guest physical
> addresses that don't belong to some guest_memfd memslot.
> 
> I think we need a broader discussion for this on where to store memory
> attributes for MMIO addresses.
> 
> I think we should at least have line of sight to storing memory
> attributes for MMIO addresses, in case we want to design something else,
> since we're putting vm_memory_attributes on a deprecation path with this
> series.

I don't know where you want to store them in KVM long term, but they
need to come from the dmabuf itself (probably via a struct
p2pdma_provider) and currently it is OK to assume all DMABUFs are
uncachable MMIO that is safe for the VM to convert into "write
combining" (eg Normal-NC on ARM)

Jason