[Xen-devel] [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

[Julien Grall <julien.grall@xxxxxxxxxx>]

For ARM64 guests, Linux is able to support either 64K or 4K page
granularity. Although, the hypercall interface is always based on 4K
page granularity.

With 64K page granularity, a single page will be spread over multiple
Xen frame.

To avoid splitting the page into 4K frame, take advantage of the
extent_order field to directly allocate/free chunk of the Linux page
size.

Note that PVMMU is only used for PV guest (which is x86) and the page
granularity is always 4KB. Some BUILD_BUG_ON has been added to ensure
that because the code has not been modified.

Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>

    Changes in v3:
        - Fix errors reported by checkpatch.pl
        - s/mfn/gfn/ based on the new naming
        - Rather than splitting the page into 4KB chunk, use the
        extent_order field to allocate directly a Linux page size. This
        is avoid lots of code for no benefits.

    Changes in v2:
        - Use xen_apply_to_page to split a page in 4K chunk
        - It's not necessary to have a smaller frame list. Re-use
        PAGE_SIZE
        - Convert reserve_additional_memory to use XEN_... macro
---
 drivers/xen/balloon.c | 47 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 9734649..c8739c8 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -70,6 +70,9 @@
 #include <xen/features.h>
 #include <xen/page.h>
 
+/* Use one extent per PAGE_SIZE to avoid break down into multiple frame */
+#define EXTENT_ORDER (fls(XEN_PFN_PER_PAGE) - 1)
+
 /*
  * balloon_process() state:
  *
@@ -230,6 +233,11 @@ static enum bp_state reserve_additional_memory(long credit)
        nid = memory_add_physaddr_to_nid(hotplug_start_paddr);
 
 #ifdef CONFIG_XEN_HAVE_PVMMU
+       /* We don't support PV MMU when Linux and Xen is using
+        * different page granularity.
+        */
+       BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
+
         /*
          * add_memory() will build page tables for the new memory so
          * the p2m must contain invalid entries so the correct
@@ -326,11 +334,11 @@ static enum bp_state reserve_additional_memory(long 
credit)
 static enum bp_state increase_reservation(unsigned long nr_pages)
 {
        int rc;
-       unsigned long  pfn, i;
+       unsigned long i;
        struct page   *page;
        struct xen_memory_reservation reservation = {
                .address_bits = 0,
-               .extent_order = 0,
+               .extent_order = EXTENT_ORDER,
                .domid        = DOMID_SELF
        };
 
@@ -352,7 +360,11 @@ static enum bp_state increase_reservation(unsigned long 
nr_pages)
                        nr_pages = i;
                        break;
                }
-               frame_list[i] = page_to_pfn(page);
+
+               /* XENMEM_populate_physmap requires a PFN based on Xen
+                * granularity.
+                */
+               frame_list[i] = xen_page_to_pfn(page);
                page = balloon_next_page(page);
        }
 
@@ -366,10 +378,15 @@ static enum bp_state increase_reservation(unsigned long 
nr_pages)
                page = balloon_retrieve(false);
                BUG_ON(page == NULL);
 
-               pfn = page_to_pfn(page);
-
 #ifdef CONFIG_XEN_HAVE_PVMMU
+               /* We don't support PV MMU when Linux and Xen is using
+                * different page granularity.
+                */
+               BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
+
                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+                       unsigned long pfn = page_to_pfn(page);
+
                        set_phys_to_machine(pfn, frame_list[i]);
 
                        /* Link back into the page tables if not highmem. */
@@ -396,14 +413,15 @@ static enum bp_state increase_reservation(unsigned long 
nr_pages)
 static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
 {
        enum bp_state state = BP_DONE;
-       unsigned long  pfn, i;
+       unsigned long i;
        struct page   *page;
        int ret;
        struct xen_memory_reservation reservation = {
                .address_bits = 0,
-               .extent_order = 0,
+               .extent_order = EXTENT_ORDER,
                .domid        = DOMID_SELF
        };
+       static struct page *pages[ARRAY_SIZE(frame_list)];
 
 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
        if (balloon_stats.hotplug_pages) {
@@ -426,7 +444,9 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
                }
                scrub_page(page);
 
-               frame_list[i] = page_to_pfn(page);
+               /* XENMEM_decrease_reservation requires a GFN */
+               frame_list[i] = xen_page_to_gfn(page);
+               pages[i] = page;
        }
 
        /*
@@ -440,12 +460,17 @@ static enum bp_state decrease_reservation(unsigned long 
nr_pages, gfp_t gfp)
 
        /* Update direct mapping, invalidate P2M, and add to balloon. */
        for (i = 0; i < nr_pages; i++) {
-               pfn = frame_list[i];
-               frame_list[i] = pfn_to_gfn(pfn);
-               page = pfn_to_page(pfn);
+               page = pages[i];
 
 #ifdef CONFIG_XEN_HAVE_PVMMU
+               /* We don't support PV MMU when Linux and Xen is using
+                * different page granularity.
+                */
+               BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
+
                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+                       unsigned long pfn = page_to_pfn(page);
+
                        if (!PageHighMem(page)) {
                                ret = HYPERVISOR_update_va_mapping(
                                                (unsigned long)__va(pfn << 
PAGE_SHIFT),
-- 
2.1.4


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