Re: [PATCH v2 4/9] KVM: Rework .gmem_invalidate() into .gmem_free_folio()

[Ackerley Tng]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

>
> [...snip...]
>
>
> -void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end)
> +void sev_gmem_free_folio(struct folio *folio)
>  {
> +	kvm_pfn_t start = page_to_pfn(folio_page(folio, 0));
> +	kvm_pfn_t end = start + (1ul << folio_order(folio));
>  	kvm_pfn_t pfn;
>
>  	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))

I thought we intended to draw the line such that the platforms don't
reference folios, and so this function should be parametrized by pfn.

I think we should still stick with

  .free_folio = kvm_gmem_free_folio

and kvm_gmem_free_folio() translates the folio to pfns and calls the
arch function, named something like .gmem_LIFECYCLE_ACTION_pfn_range.

Now for LIFECYCLE_ACTION, one way to think of it is that this should
represent the point in the lifecycle of guest_memfd memory where the
memory is removed from guest's private use, so perhaps "host_reclaim"?

Then kvm_gmem_free_folio() becomes:

   kvm_gmem_free_folio() {
       pfn_start, pfn_end = translate folio to pfn range;
       kvm_x86_call(gmem_host_reclaim_pfn_range)(pfn_start, pfn_end);
   }


And in conversions

   if (!to_private) {
       pfn_start, pfn_end = translate guest_memfd offset range to pfns;
       kvm_x86_call(gmem_host_reclaim_pfn_range)(pfn_start, pfn_end);
   }

(and now it is right for the !to_private check to remain in guest_memfd
since we're explicitly using that to guard a *host* reclaim function.

Another way to think of LIFECYCLE_ACTION is to have the hook represent
the point of time where the memory's attribute is being set. Perhaps
"set_attributes"?

Then kvm_gmem_free_folio() becomes:

   kvm_gmem_free_folio() {
       pfn_start, pfn_end = translate folio to pfn range;
       kvm_x86_call(gmem_set_attributes_pfn_range)(pfn_start, pfn_end, SHARED);
   }

because when freeing a folio we want to set the attributes to SHARED.

And in conversions

   if (kvm_x86_call(gmem_should_set_attributes_pfn_range)(SHARED)) {
       pfn_start, pfn_end = translate guest_memfd offset range to pfns;
       kvm_x86_call(gmem_set_attributes_pfn_range)(pfn_start, pfn_end, SHARED);
   }

This is biased towards conversions (related proposal at [1]).

[1] https://lore.kernel.org/all/CAEvNRgGX3GkazCWM=6y9YLgn=YemXuG==Oo+L58cac1Fd86_TQ@xxxxxxxxxxxxxx/

>
> [...snip...]
>