mshv: support 1G hugepages by passing them as 2M-aligned chunks

The hypervisor's map GPA hypercall coalesces contiguous 2M-aligned
chunks into 1G mappings when alignment permits, so the driver can
support 1G hugepages by feeding them in as 2M chunks. Note that this
is the only way to make 1G mappings; there is no way to directly map
a 1G hugepage using the hypercall.

Always emit a 2M (PMD_ORDER) stride for the huge-page case. The
hypercall has no 1G stride, so 1G folios are processed as a
sequence of 2M chunks. Folios whose order is less than PMD_ORDER
(e.g. mTHP) fall back to single-page stride; mapping them as 2M
would fail in the hypervisor anyway.

Assisted-by: Copilot-CLI:claude-opus-4.7
Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Acked-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/drivers/hv/mshv_regions.c b/drivers/hv/mshv_regions.c
index fdffd4f002f6fdc4bc435426b81a075dc7c9c836..6d65e5b4215295afa20f8594bf6d965b44e4fcbe 100644 (file)
--- a/drivers/hv/mshv_regions.c
+++ b/drivers/hv/mshv_regions.c
@@ -29,29 +29,27 @@
  * Uses huge page stride if the backing page is huge and the guest mapping
  * is properly aligned; otherwise falls back to single page stride.
  *
- * Return: Stride in pages, or -EINVAL if page order is unsupported.
+ * Return: Stride in pages.
  */
-static int mshv_chunk_stride(struct page *page,
-                            u64 gfn, u64 page_count)
+static unsigned int mshv_chunk_stride(struct page *page, u64 gfn,
+                                     u64 page_count)
 {
-       unsigned int page_order;
+       unsigned int page_order = folio_order(page_folio(page));
 
        /*
         * Use single page stride by default. For huge page stride, the
-        * page must be compound and point to the head of the compound
-        * page, and both gfn and page_count must be huge-page aligned.
+        * folio order must be at least PMD_ORDER, the page's PFN must be
+        * 2M-aligned (so that a 2M-aligned tail page of a larger folio is
+        * acceptable), and both gfn and page_count must be 2M-aligned.
         */
-       if (!PageCompound(page) || !PageHead(page) ||
+       if (page_order < PMD_ORDER ||
+           !IS_ALIGNED(page_to_pfn(page), PTRS_PER_PMD) ||
            !IS_ALIGNED(gfn, PTRS_PER_PMD) ||
            !IS_ALIGNED(page_count, PTRS_PER_PMD))
                return 1;
 
-       page_order = folio_order(page_folio(page));
-       /* The hypervisor only supports 2M huge page */
-       if (page_order != PMD_ORDER)
-               return -EINVAL;
-
-       return 1 << page_order;
+       /* Use 2M stride always i.e. process 1G folios as 2M chunks */
+       return 1 << PMD_ORDER;
 }
 
 /**
@@ -86,15 +84,14 @@ static long mshv_region_process_chunk(struct mshv_mem_region *region,
        u64 gfn = region->start_gfn + page_offset;
        u64 count;
        struct page *page;
-       int stride, ret;
+       unsigned int stride;
+       int ret;
 
        page = region->mreg_pages[page_offset];
        if (!page)
                return -EINVAL;
 
        stride = mshv_chunk_stride(page, gfn, page_count);
-       if (stride < 0)
-               return stride;
 
        /* Start at stride since the first stride is validated */
        for (count = stride; count < page_count; count += stride) {