linux-kernel - Re: [RFC PATCH 26/39] KVM: guest_memfd: Track faultability within a struct kvm_gmem

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Message-ID: <20241016235424.GU3559746@nvidia.com>
Date: Wed, 16 Oct 2024 20:54:24 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.com>,
	Ackerley Tng <ackerleytng@...gle.com>, tabba@...gle.com,
	quic_eberman@...cinc.com, roypat@...zon.co.uk, rientjes@...gle.com,
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	pgonda@...gle.com, oliver.upton@...ux.dev,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 26/39] KVM: guest_memfd: Track faultability within a
 struct kvm_gmem_private

On Wed, Oct 16, 2024 at 07:49:31PM -0400, Peter Xu wrote:
> On Wed, Oct 16, 2024 at 07:51:57PM -0300, Jason Gunthorpe wrote:
> > On Wed, Oct 16, 2024 at 04:16:17PM -0400, Peter Xu wrote:
> > > 
> > > Is there chance that when !CoCo will be supported, then external modules
> > > (e.g. VFIO) can reuse the old user mappings, just like before gmemfd?
> > > 
> > > To support CoCo, I understand gmem+offset is required all over the places.
> > > However in a non-CoCo context, I wonder whether the other modules are
> > > required to stick with gmem+offset, or they can reuse the old VA ways,
> > > because how it works can fundamentally be the same as before, except that
> > > the folios now will be managed by gmemfd.
> > 
> > My intention with iommufd was to see fd + offest as the "new" way
> > to refer to all guest memory and discourage people from using VMA
> > handles.
> 
> Does it mean anonymous memory guests will not be supported at all for
> iommufd?

No, they can use the "old" way with normal VMA's still, or they can
use an anonymous memfd with the new way..

I just don't expect to have new complex stuff built on the VMA
interface - I don't expect guestmemfd VMAs to work.

Jason