Re: [PATCH v2 14/14] KVM: guest_memfd: abstract how prepared folios are recorded

[Michael Roth <michael.roth@amd.com>]

On Fri, Jul 26, 2024 at 02:51:57PM -0400, Paolo Bonzini wrote:
> Right now, large folios are not supported in guest_memfd, and therefore the order
> used by kvm_gmem_populate() is always 0.  In this scenario, using the up-to-date
> bit to track prepared-ness is nice and easy because we have one bit available
> per page.
> 
> In the future, however, we might have large pages that are partially populated;
> for example, in the case of SEV-SNP, if a large page has both shared and private
> areas inside, it is necessary to populate it at a granularity that is smaller
> than that of the guest_memfd's backing store.  In that case we will have
> to track preparedness at a 4K level, probably as a bitmap.
> 
> In preparation for that, do not use explicitly folio_test_uptodate() and
> folio_mark_uptodate().  Return the state of the page directly from
> __kvm_gmem_get_pfn(), so that it is expected to apply to 2^N pages
> with N=*max_order.  The function to mark a range as prepared for now
> takes just a folio, but is expected to take also an index and order
> (or something like that) when large pages are introduced.
> 
> Thanks to Michael Roth for pointing out the issue with large pages.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Michael Roth <michael.roth@amd.com>