Re: [PATCH v1 00/13] KVM: Introduce KVM Userfault

Peter Xu
2025-01-16T23:17:15Z

On Thu, Jan 16, 2025 at 03:04:45PM -0800, James Houghton wrote:
> On Thu, Jan 16, 2025 at 2:16 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, Jan 16, 2025, Peter Xu wrote:
> > > On Thu, Jan 16, 2025 at 03:19:49PM -0500, Peter Xu wrote:
> > > > > For the gmem case, userfaultfd cannot be used, so KVM Userfault isn't
> > > > > replacing it. And as of right now anyway, KVM Userfault *does* provide
> > > > > a complete post-copy system for gmem.
> > > > >
> > > > > When gmem pages can be mapped into userspace, for post-copy to remain
> > > > > functional, userspace-mapped gmem will need userfaultfd integration.
> > > > > Keep in mind that even after this integration happens, userfaultfd
> > > > > alone will *not* be a complete post-copy solution, as vCPU faults
> > > > > won't be resolved via the userspace page tables.
> > > >
> > > > Do you know in context of CoCo, whether a private page can be accessed at
> > > > all outside of KVM?
> > > >
> > > > I think I'm pretty sure now a private page can never be mapped to
> > > > userspace. However, can another module like vhost-kernel access it during
> > > > postcopy? My impression of that is still a yes, but then how about
> > > > vhost-user?
> > > >
> > > > Here, the "vhost-kernel" part represents a question on whether private
> > > > pages can be accessed at all outside KVM. While "vhost-user" part
> > > > represents a question on whether, if the previous vhost-kernel question
> > > > answers as "yes it can", such access attempt can happen in another
> > > > process/task (hence, not only does it lack KVM context, but also not
> > > > sharing the same task context).
> > >
> > > Right after I sent it, I just recalled whenever a device needs to access
> > > the page, it needs to be converted to shared pages first..
> >
> > FWIW, once Trusted I/O comes along, "trusted" devices will be able to access guest
> > private memory. The basic gist is that the IOMMU will enforce access to private
> > memory, e.g. on AMD the IOMMU will check the RMP[*], and I believe the plan for
> > TDX is to have the IOMMU share the Secure-EPT tables that are used by the CPU.
> >
> > [*] https://www.amd.com/content/dam/amd/en/documents/developer/sev-tio-whitepaper.pdf

Thanks, Sean. This is interesting to know..

>
> Hi Sean,
>
> Do you know what API the IOMMU driver would use to get the private
> pages to map? Normally it'd use GUP, but GUP would/should fail for
> guest-private pages, right?

James,

I'm still reading the link Sean shared, looks like there's answer in the
white paper on this on assigned devices:

TDIs access memory via either guest virtual address (GVA) space or
guest physical address (GPA) space. The I/O Memory Management Unit
(IOMMU) in the host hardware is responsible for translating the
provided GVAs or GPAs into system physical addresses
(SPAs). Because SEV-SNP enforces access control at the time of
translation, the IOMMU performs RMP entry lookups on translation

So I suppose after the device is attested and trusted, it can directly map
everything if wanted, and DMA directly to the encrypted pages.

OTOH, for my specific question (on vhost-kernel, or vhost-user), I suppose
they cannot be attested but still be part of host software.. so I'm
guessing they'll need to still stick with shared pages, and use a bounce
buffer to do DMAs..

--
Peter Xu