Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

From: Andy Lutomirski
Date: Fri Sep 09 2022 - 00:48:52 EST

On 8/19/22 17:27, Kirill A. Shutemov wrote:
On Thu, Aug 18, 2022 at 08:00:41PM -0700, Hugh Dickins wrote:
On Thu, 18 Aug 2022, Kirill A . Shutemov wrote:
On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:

If your memory could be swapped, that would be enough of a good reason
to make use of shmem.c: but it cannot be swapped; and although there
are some references in the mailthreads to it perhaps being swappable
in future, I get the impression that will not happen soon if ever.

If your memory could be migrated, that would be some reason to use
filesystem page cache (because page migration happens to understand
that type of memory): but it cannot be migrated.

Migration support is in pipeline. It is part of TDX 1.5 [1]. And swapping
theoretically possible, but I'm not aware of any plans as of now.


I always forget, migration means different things to different audiences.
As an mm person, I was meaning page migration, whereas a virtualization
person thinks VM live migration (which that reference appears to be about),
a scheduler person task migration, an ornithologist bird migration, etc.

But you're an mm person too: you may have cited that reference in the
knowledge that TDX 1.5 Live Migration will entail page migration of the
kind I'm thinking of. (Anyway, it's not important to clarify that here.)

TDX 1.5 brings both.

In TDX speak, mm migration called relocation. See TDH.MEM.PAGE.RELOCATE.

This seems to be a pretty bad fit for the way that the core mm migrates pages. The core mm unmaps the page, then moves (in software) the contents to a new address, then faults it in. TDH.MEM.PAGE.RELOCATE doesn't fit into that workflow very well. I'm not saying it can't be done, but it won't just work.