Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls

[Ackerley Tng]

Xu Yilun <yilun.xu@xxxxxxxxxxxxxxx> writes:

> On Wed, Jul 16, 2025 at 03:22:06PM -0700, Ackerley Tng wrote:
>> Yan Zhao <yan.y.zhao@xxxxxxxxx> 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@xxxxxxxx> 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.

I see, thanks for explaining.

> 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.
>

If by the time KVM gets the conversion request, the page is unpinned,
then we're all good, right?

When guest_memfd gets the conversion request, as part of conversion
handling it will request to zap the page from stage-2 page tables. TDX
module would see that the page is unpinned and the unmapping will
proceed fine. Is that understanding correct?

>> 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.
>

On this note, I'd also like to check something else. Putting TDX connect
and IOMMUs aside, if the host unmaps a guest private page today without
the guest requesting it, the unmapping will work and the guest will be
broken, right?

> 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".