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

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Message-ID: <98eacabd-3465-4f5d-b045-8e8595d68676@redhat.com>
Date: Thu, 17 Oct 2024 16:56:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...dia.com>
Cc: Ackerley Tng <ackerleytng@...gle.com>, tabba@...gle.com,
 quic_eberman@...cinc.com, roypat@...zon.co.uk, rientjes@...gle.com,
 fvdl@...gle.com, jthoughton@...gle.com, seanjc@...gle.com,
 pbonzini@...hat.com, zhiquan1.li@...el.com, fan.du@...el.com,
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 jhubbard@...dia.com, willy@...radead.org, shuah@...nel.org,
 brauner@...nel.org, bfoster@...hat.com, kent.overstreet@...ux.dev,
 pvorel@...e.cz, rppt@...nel.org, richard.weiyang@...il.com,
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 vkuznets@...hat.com, maciej.wieczor-retman@...el.com, 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 17.10.24 01:49, 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?
> 
> Indeed it's very rare now, lose quite some flexibility (v.s. fd based), and
> I can't think of a lot besides some default configs or KSM users (which I
> would expect rare), but still I wonder there're other use cases that people
> would still need to stick with anon, hence fd isn't around.

Not sure I completely understand the question, but for most VMs out 
there I expect an anonymous memory to remain the default memory backing.

Regarding users of iommufd, I have absolutely no clue :)

-- 
Cheers,

David / dhildenb