On 26.07.24 08:55, Patrick Roy wrote:
On Mon, 2024-07-22 at 13:28 +0100, "Vlastimil Babka (SUSE)" wrote:
=== Implementation ===
This patch series introduces a new flag to the `KVM_CREATE_GUEST_MEMFD`
to remove its pages from the direct map when they are allocated. When
trying to run a guest from such a VM, we now face the problem that
without either userspace or kernelspace mappings of guest_memfd, KVM
cannot access guest memory to, for example, do MMIO emulation of access
memory used to guest/host communication. We have multiple options for
solving this when running non-CoCo VMs: (1) implement a TDX-light
solution, where the guest shares memory that KVM needs to access, and
relies on paravirtual solutions where this is not possible (e.g. MMIO),
(2) have KVM use userspace mappings of guest_memfd (e.g. a
memfd_secret-style solution), or (3) dynamically reinsert pages into the
direct map whenever KVM wants to access them.
This RFC goes for option (3). Option (1) is a lot of overhead for very
little gain, since we are not actually constrained by a physical
inability to access guest memory (e.g. we are not in a TDX context where
accesses to guest memory cause a #MC). Option (2) has previously been
rejected [1].
Do the pages have to have the same address when they are temporarily mapped?
Wouldn't it be easier to do something similar to kmap_local_page() used for
HIMEM? I.e. you get a temporary kernel mapping to do what's needed, but it
doesn't have to alter the shared directmap.
Maybe that was already discussed somewhere as unsuitable but didn't spot it
here.
For what I had prototyped here, there's no requirement to have the pages
mapped at the same address (I remember briefly looking at memremap to
achieve the temporary mappings, but since that doesnt work for normal
memory, I gave up on that path). However, I think guest_memfd is moving
into a direction where ranges marked as "in-place shared" (e.g. those
that are temporarily reinserted into the direct map in this RFC) should
be able to be GUP'd [1]. I think for that the direct map entries would
need to be present, right?
Yes, we'd allow GUP. Of course, one could think of a similar extension
like secretmem that would allow shared memory to get mapped into user
page tables but would disallow any GUP on (shared) guest_memfd memory.
--
Cheers,
David / dhildenb
