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

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Message-ID: <aHjDIxxbv0DnqI6S@yilunxu-OptiPlex-7050>
Date: Thu, 17 Jul 2025 17:32:19 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: Yan Zhao <yan.y.zhao@...el.com>,
	Vishal Annapurve <vannapurve@...gle.com>,
	Jason Gunthorpe <jgg@...pe.ca>, Alexey Kardashevskiy <aik@....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 16, 2025 at 03:22:06PM -0700, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@...el.com> writes:
> 
> > 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.
> > 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.
> >
> >
> 
> Yan, Yilun, would it work if, on conversion,
> 
> 1. guest_memfd notifies IOMMU that a conversion is about to happen for a
>    PFN range

It is the Guest fw call to release the pinning. By the time VMM get the
conversion requirement, the page is already physically unpinned. So I
agree with Jason the pinning doesn't have to reach to iommu from SW POV.

> 2. IOMMU forwards the notification to TDX code in the kernel
> 3. TDX code in kernel tells TDX module to stop thinking of any PFNs in
>    the range as pinned for DMA?

TDX host can't stop the pinning. Actually this mechanism is to prevent
host from unpin/unmap the DMA out of Guest expectation.

Thanks,
Yilun

> 
> If the above is possible then by the time we get to unmapping from
> S-EPTs, TDX module would already consider the PFNs in the range "not
> pinned for DMA".