linux-kernel - Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce KVM_GMEM_CONVERT

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Message-ID: <CAGtprH9-njcgQjGZvGbbVX+i8D-qPUOkKFHbOWA20962niLTcw@mail.gmail.com>
Date: Wed, 2 Jul 2025 06:54:10 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Alexey Kardashevskiy <aik@....com>, Fuad Tabba <tabba@...gle.com>, 
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Subject: Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce
 KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls

On Wed, Jul 2, 2025 at 1:38 AM Yan Zhao <yan.y.zhao@...el.com> wrote:
>
> On Tue, Jun 24, 2025 at 07:10:38AM -0700, Vishal Annapurve wrote:
> > On Tue, Jun 24, 2025 at 6:08 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > >
> > > On Tue, Jun 24, 2025 at 06:23:54PM +1000, Alexey Kardashevskiy wrote:
> > >
> > > > Now, I am rebasing my RFC on top of this patchset and it fails in
> > > > kvm_gmem_has_safe_refcount() as IOMMU holds references to all these
> > > > folios in my RFC.
> > > >
> > > > So what is the expected sequence here? The userspace unmaps a DMA
> > > > page and maps it back right away, all from the userspace? The end
> > > > result will be the exactly same which seems useless. And IOMMU TLB
> >
> >  As Jason described, ideally IOMMU just like KVM, should just:
> > 1) Directly rely on guest_memfd for pinning -> no page refcounts taken
> > by IOMMU stack
> In TDX connect, TDX module and TDs do not trust VMM. So, it's the TDs to inform
> TDX module about which pages are used by it for DMAs purposes.
> So, if a page is regarded as pinned by TDs for DMA, the TDX module will fail the
> unmap of the pages from S-EPT.
>
> If IOMMU side does not increase refcount, IMHO, some way to indicate that
> certain PFNs are used by TDs for DMA is still required, so guest_memfd can
> reject the request before attempting the actual unmap.

So it looks like guest_memfd will need an interface with KVM/IOMMU
backends to check if unmapping can succeed. And if unmapping still
fails, there should be a way for KVM/IOMMU backends to kill the TD and
any TDIs bound to that TD.

> Otherwise, the unmap of TD-DMA-pinned pages will fail.
>
> Upon this kind of unmapping failure, it also doesn't help for host to retry
> unmapping without unpinning from TD.
>