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

Peter Xu
2025-01-16T20:32:15Z

On Thu, Jan 16, 2025 at 03:19:49PM -0500, Peter Xu wrote:
> James,
>
> Sorry for a late reply.
>
> I still do have one or two pure questions, but nothing directly relevant to
> your series.
>
> On Thu, Jan 02, 2025 at 12:53:11PM -0500, James Houghton wrote:
> > So I'm not pushing for KVM Userfault to replace userfaultfd; it's not
> > worth the extra/duplicated complexity. And at LPC, Paolo and Sean
> > indicated that this direction was indeed wrong. I have another way to
> > make this work in mind. :)
>
> Do you still want to share it, more or less? :)
>
> >
> > 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..

So I suppose the questions were not valid at all! It is not about the
context but that the pages will be shared always whenever a device in
whatever form will access it..

Fundamentally I'm thinking about whether userfaultfd must support (fd,
offset) tuple. Now I suppose it's not, because vCPUs accessing
private/shared will all exit to userspace, while all non-vCPU / devices can
access shared pages only.

In that case, looks like userfaultfd can support CoCo on device emulations
by sticking with virtual-address traps like before, at least from that
specific POV.

--
Peter Xu