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

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Message-ID: <Z4lp5QzdOX0oYGOk@x1n>
Date: Thu, 16 Jan 2025 15:19:49 -0500
From: Peter Xu <peterx@...hat.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Yan Zhao <yan.y.zhao@...el.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Anish Moorthy <amoorthy@...gle.com>,
	Peter Gonda <pgonda@...gle.com>,
	David Matlack <dmatlack@...gle.com>, Wei W <wei.w.wang@...el.com>,
	kvm@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev
Subject: Re: [PATCH v1 00/13] KVM: Introduce KVM Userfault

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).

Thanks,

-- 
Peter Xu