Message-ID: <20260203181321.GX2328995@ziepe.ca>
Date: Tue, 3 Feb 2026 14:13:21 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alexey Kardashevskiy <aik@....com>
Cc: Sean Christopherson <seanjc@...gle.com>,
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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 Tue, Feb 03, 2026 at 12:07:46PM +1100, Alexey Kardashevskiy wrote:
> On 29/1/26 12:16, Jason Gunthorpe wrote:
> > On Wed, Jan 28, 2026 at 05:03:27PM -0800, Sean Christopherson wrote:
> > 
> > > For a dmabuf fd, the story is the same as guest_memfd.  Unless private vs. shared
> > > is all or nothing, and can never change, then the only entity that can track that
> > > info is the owner of the dmabuf.  And even if the private vs. shared attributes
> > > are constant, tracking it external to KVM makes sense, because then the provider
> > > can simply hardcode %true/%false.
> > 
> > Oh my I had not given that bit any thought. My remarks were just about
> > normal non-CC systems.
> > 
> > So MMIO starts out shared, and then converts to private when the guest
> > triggers it. It is not all or nothing, there are permanent shared
> > holes in the MMIO ranges too.
> > 
> > Beyond that I don't know what people are thinking.
> > 
> > Clearly VFIO has to revoke and disable the DMABUF once any of it
> > becomes private.
> 
> huh? Private MMIO still has to be mapped in the NPT (well, on
> AMD). It is the userspace mapping which we do not want^wneed and we
> do not by using dmabuf.

Well, we don't know what the DMABUF got imported into, so the non-KVM
importers using the shared mapping certainly have to drop it.

How exactly to make that happen is going to be interesting..

Jason

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