Re: [RFC PATCH v5 02/38] KVM: arm64: Add lock/unlock memslot user API

[Marc Zyngier <maz@kernel.org>]

On Tue, 15 Feb 2022 11:03:59 +0000,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> > If a memslot with read/write permission is locked with read only,
> > and then unlocked, can userspace expect stage 2 mapping for the
> > memslot to be updated with read/write ?
> 
> Locking a memslot with the read flag would map the memory described by the
> memslot with read permissions at stage 2. When the memslot is unlocked, KVM
> won't touch the stage 2 entries.
> 
> When the memslot is unlocked, the pages (as in, struct page) backing the VM
> memory as described by the memslot are unpinned. Then the host's MM subsystem
> can treat the memory like any other pages (make them old, new, unmap them, do
> nothing, etc), and the MMU notifier will take care of updating the stage 2
> entries as necessary.
> 
> I guess I should have been more precise in the description. I'll
> change "causes the memory pinned when locking the memslot specified
> in args[0] to be unpinned" to something that clearly states that the
> memory in the host that backs the memslot is unpinned.
> 
> > Can userspace delete the memslot that is locked (without unlocking) ?
> 
> No, it cannot.
> 
> > If so, userspace can expect the corresponding range to be implicitly
> > unlocked, correct ?
> 
> Userspace must explicitely unlock the memslot before deleting it. I want
> userspace to be explicit in its intent.

Does it get in the way of making this robust wrt userspace being
killed (or terminating without unlock first)?

	M.

-- 
Without deviation from the norm, progress is not possible.
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