Re: [PATCH v5 00/13] KVM: mm: fd-based approach for supporting KVM guest private memory

[Andy Lutomirski]

On Fri, Apr 1, 2022, at 7:59 AM, Quentin Perret wrote:
> On Thursday 31 Mar 2022 at 09:04:56 (-0700), Andy Lutomirski wrote:


> To answer your original question about memory 'conversion', the key
> thing is that the pKVM hypervisor controls the stage-2 page-tables for
> everyone in the system, all guests as well as the host. As such, a page
> 'conversion' is nothing more than a permission change in the relevant
> page-tables.
>

So I can see two different ways to approach this.

One is that you split the whole address space in half and, just like SEV and 
TDX, allocate one bit to indicate the shared/private status of a page.  This 
makes it work a lot like SEV and TDX.

The other is to have shared and private pages be distinguished only by their 
hypercall history and the (protected) page tables.  This saves some address 
space and some page table allocations, but it opens some cans of worms too.  In 
particular, the guest and the hypervisor need to coordinate, in a way that the 
guest can trust, to ensure that the guest's idea of which pages are private 
match the host's.  This model seems a bit harder to support nicely with the 
private memory fd model, but not necessarily impossible.

Also, what are you trying to accomplish by having the host userspace mmap 
private pages?  Is the idea that multiple guest could share the same page until 
such time as one of them tries to write to it?  That would be kind of like 
having a third kind of memory that's visible to host and guests but is 
read-only for everyone.  TDX and SEV can't support this at all (a private page 
belongs to one guest and one guest only, at least in SEV and in the current TDX 
SEAM spec).  I imagine that this could be supported with private memory fds 
with some care without mmap, though -- the host could still populate the page 
with memcpy.  Or I suppose a memslot could support using MAP_PRIVATE fds and 
have approximately the right semantics.

--Andy