Sean Christopherson <[email protected]> writes:
> On Thu, Aug 13, 2026, Binbin Wu wrote:
>> On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:
>> > +#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
>> > +static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start,
>> > pgoff_t end)
>> > +{
>> > + struct folio_batch fbatch;
>> > + pgoff_t next = start;
>> > + int i;
>> > +
>> > + folio_batch_init(&fbatch);
>> > + while (filemap_get_folios(inode->i_mapping, &next, end - 1, &fbatch)) {
>> > + for (i = 0; i < folio_batch_count(&fbatch); ++i) {
>> > + struct folio *folio = fbatch.folios[i];
>> > + pgoff_t start_index, end_index;
>> > + kvm_pfn_t start_pfn;
>> > + kvm_pfn_t nr_pages;
>> > +
>> > + start_index = max(start, folio->index);
>> > + end_index = min(end, folio_next_index(folio));
>> > + /*
>> > + * end_index is either in folio or points to
>> > + * the first page of the next folio. Hence,
>> > + * all pages in range [start_index, end_index)
>> > + * are contiguous.
>> > + */
>> > + start_pfn = folio_file_pfn(folio, start_index);
>> > + nr_pages = end_index - start_index;
>> > +
>> > + kvm_arch_gmem_make_shared(start_pfn, nr_pages);
>> > + }
>> > +
>> > + folio_batch_release(&fbatch);
>> > + cond_resched();
>> > + }
>> > +}
>> > +#else
>> > +static void kvm_gmem_make_shared(struct inode *inode, pgoff_t start,
>> > pgoff_t end) {}
>> > +#endif
>> > +
>> > static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start,
>> > size_t nr_pages, uint64_t attrs,
>> > pgoff_t *err_index)
>> > @@ -599,7 +636,12 @@ static int __kvm_gmem_set_attributes(struct inode
>> > *inode, pgoff_t start,
>> >
>> > filter = to_private ? KVM_FILTER_SHARED : KVM_FILTER_PRIVATE;
>> > kvm_gmem_invalidate_start(inode, start, end, filter);
>> > +
>> > + if (!to_private)
Should I make this condition
if (!to_private && kvm_x86_ops.gmem_make_shared)
instead?
Will that help?
>> > + kvm_gmem_make_shared(inode, start, end);
>>
>> If both KVM_AMD_SEV and KVM_INTEL_TDX are enabled, HAVE_KVM_ARCH_GMEM_CONVERT
>> will be enabled and the logic in kvm_gmem_make_shared() introduces
>> unnecessary
>> overhead for TDX.
>> Not sure about CSPs, but in a standard distribution kernel, it's very likely
>> that both are enabled, right？
>
> Yep.
>
>> Should kvm_gmem_make_shared() do some optimization or the overhead is
>> relative
>> small in the conversion to shared path so that the optimization is not worth
>> it?
>
> I assume it's relatively small, but it's probably a good idea to gets numbers
> so
> that we can make a semi-informed decision. E.g. if processing 1GiB work of
> 4KiB
> folios is <100 cycles, I think we generally don't care. But if it's more like
> tens of microseconds, then I think we do care enough to optimize the walking
> for
> TDX.
