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

[Kirill A . Shutemov]

On Thu, Sep 08, 2022 at 09:48:35PM -0700, Andy Lutomirski wrote:
> 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.
> > > > 
> > > > [1] 
> > > > https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html
> > > 
> > > 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.

Hm. From what I see we have all necessary infrastructure in place.

Unmaping is NOP for inaccessible pages as it is never mapped and we have
mapping->a_ops->migrate_folio() callback that allows to replace software
copying with whatever is needed, like TDH.MEM.PAGE.RELOCATE.

What do I miss?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov