[Xen-devel] [PATCH v2 3/3] xen/arm: fix mask calculation in pdx_init_mask

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

The mask calculation in pdx_init_mask is wrong when the first bank
starts at address 0x0. The reason is that pdx_init_mask will do '0 - 1'
causing an underflow. As a result, the mask becomes 0xffffffffffffffff
which is the biggest possible mask and ends up causing a significant
memory waste in the frametable size computation.

For instance, on platforms that have a low memory bank starting at 0x0
and a high memory bank, the frametable will end up covering all the
holes in between.

The purpose of the mask is to be passed as a parameter to
pfn_pdx_hole_setup, which based on the mask parameter calculates
pfn_pdx_hole_shift, pfn_pdx_bottom_mask, etc. which are actually the
important masks for frametable initialization later on.

pfn_pdx_hole_setup never compresses addresses below MAX_ORDER bits (1GB
on ARM). Thus, it is safe to initialize mask passing 1ULL << (MAX_ORDER
+ PAGE_SHIFT) as start address to pdx_init_mask.

Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
CC: JBeulich@xxxxxxxx
CC: andrew.cooper3@xxxxxxxxxx
CC: George.Dunlap@xxxxxxxxxxxxx
CC: ian.jackson@xxxxxxxxxxxxx
CC: konrad.wilk@xxxxxxxxxx
CC: tim@xxxxxxx
CC: wei.liu2@xxxxxxxxxx
---

Changes in v2:
- update commit message
- add in-code comments regarding update sites
- improve in-code comments
- move the mask initialization changes to pdx_init_mask
---
 xen/arch/arm/setup.c | 9 ++++++++-
 xen/common/pdx.c     | 8 +++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index ccb0f181ea..afaafe7b84 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -482,7 +482,14 @@ static void __init init_pdx(void)
 {
     paddr_t bank_start, bank_size, bank_end;
 
-    u64 mask = pdx_init_mask(bootinfo.mem.bank[0].start);
+    /*
+     * Pass 0x0 to pdx_init_mask to get a mask initialized with the
+     * first to 1<<MAX_ORDER pages of RAM left uncompressed.
+     *
+     * If the logic changes in pfn_pdx_hole_setup we might have to
+     * update this function too.
+     */
+    u64 mask = pdx_init_mask(0x0);
     int bank;
 
     for ( bank = 0 ; bank < bootinfo.mem.nr_banks; bank++ )
diff --git a/xen/common/pdx.c b/xen/common/pdx.c
index bb7e437049..268d6f7ec3 100644
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -50,9 +50,13 @@ static u64 __init fill_mask(u64 mask)
     return mask;
 }
 
+/*
+ * We always map the first 1<<MAX_ORDER pages of RAM, hence, they
+ * are left uncompressed.
+ */
 u64 __init pdx_init_mask(u64 base_addr)
 {
-    return fill_mask(base_addr - 1);
+    return fill_mask(max(base_addr, (u64)1 << (MAX_ORDER + PAGE_SHIFT)) - 1);
 }
 
 u64 __init pdx_region_mask(u64 base, u64 len)
@@ -80,6 +84,8 @@ void __init pfn_pdx_hole_setup(unsigned long mask)
      * This guarantees that page-pointer arithmetic remains valid within
      * contiguous aligned ranges of 2^MAX_ORDER pages. Among others, our
      * buddy allocator relies on this assumption.
+     *
+     * If the logic changes here, we might have to update init_pdx too.
      */
     for ( j = MAX_ORDER-1; ; )
     {
-- 
2.17.1


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