Re: [PATCH 6/8] KVM: gmem, x86: Add gmem hook for invalidating private memory

[Xu Yilun]

On 2023-08-15 at 10:18:53 -0700, isaku.yamahata@xxxxxxxxx wrote:
> From: Michael Roth <michael.roth@xxxxxxx>
> 
> TODO: add a CONFIG option that can be to completely skip arch
> invalidation loop and avoid __weak references for arch/platforms that
> don't need an additional invalidation hook.
> 
> In some cases, like with SEV-SNP, guest memory needs to be updated in a
> platform-specific manner before it can be safely freed back to the host.
> Add hooks to wire up handling of this sort when freeing memory in
> response to FALLOC_FL_PUNCH_HOLE operations.
> 
> Also issue invalidations of all allocated pages when releasing the gmem
> file so that the pages are not left in an unusable state when they get
> freed back to the host.
> 
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> Link: https://lore.kernel.org/r/20230612042559.375660-3-michael.roth@xxxxxxx
>

[...]
 
> +/* Handle arch-specific hooks needed before releasing guarded pages. */
> +static void kvm_gmem_issue_arch_invalidate(struct kvm *kvm, struct file *file,
> +					   pgoff_t start, pgoff_t end)
> +{
> +	pgoff_t file_end = i_size_read(file_inode(file)) >> PAGE_SHIFT;
> +	pgoff_t index = start;
> +
> +	end = min(end, file_end);
> +
> +	while (index < end) {
> +		struct folio *folio;
> +		unsigned int order;
> +		struct page *page;
> +		kvm_pfn_t pfn;
> +
> +		folio = __filemap_get_folio(file->f_mapping, index,
> +					    FGP_LOCK, 0);
> +		if (!folio) {
> +			index++;
> +			continue;
> +		}
> +
> +		page = folio_file_page(folio, index);
> +		pfn = page_to_pfn(page);
> +		order = folio_order(folio);
> +
> +		kvm_arch_gmem_invalidate(kvm, pfn, pfn + min((1ul << order), end - index));

Observed an issue there.

The valid page may not point to the first page in the folio, then the
range [pfn, pfn + (1ul << order)) expands to the next folio. This makes
a part of the pages be invalidated again when loop to the next folio.

On TDX, it causes TDH_PHYMEM_PAGE_WBINVD failed.

> +
> +		index = folio_next_index(folio);
> +		folio_unlock(folio);
> +		folio_put(folio);
> +
> +		cond_resched();
> +	}
> +}

My fix would be:

diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
index e629782d73d5..3665003c3746 100644
--- a/virt/kvm/guest_mem.c
+++ b/virt/kvm/guest_mem.c
@@ -155,7 +155,7 @@ static void kvm_gmem_issue_arch_invalidate(struct kvm *kvm, struct inode *inode,

        while (index < end) {
                struct folio *folio;
-               unsigned int order;
+               pgoff_t ntails;
                struct page *page;
                kvm_pfn_t pfn;

@@ -168,9 +168,9 @@ static void kvm_gmem_issue_arch_invalidate(struct kvm *kvm, struct inode *inode,

                page = folio_file_page(folio, index);
                pfn = page_to_pfn(page);
-               order = folio_order(folio);
+               ntails = folio_nr_pages(folio) - folio_page_idx(folio, page);

-               kvm_arch_gmem_invalidate(kvm, pfn, pfn + min((1ul << order), end - index));
+               kvm_arch_gmem_invalidate(kvm, pfn, pfn + min(ntails, end - index));

                index = folio_next_index(folio);
                folio_unlock(folio);

Thanks,
Yilun