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

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Message-ID: <Z4lsxgFSdiqpNtdG@x1n>
Date: Thu, 16 Jan 2025 15:32:06 -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

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