[Xen-changelog] [xen staging] xen/arm: fix mask calculation in pdx_init_mask

[patchbot@xxxxxxx]

commit 11911563610786615c2b3a01cdcaaf09a6f9e38d
Author:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
AuthorDate: Fri Jun 21 13:20:25 2019 -0700
Commit:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
CommitDate: Fri Jun 21 14:07:47 2019 -0700

    xen/arm: fix mask calculation in pdx_init_mask
    
    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>
    Reviewed-by: Julien Grall <julien.grall@xxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/arm/setup.c | 9 ++++++++-
 xen/common/pdx.c     | 7 ++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
index d5d188a105..215746a5c3 100644
--- a/xen/arch/arm/setup.c
+++ b/xen/arch/arm/setup.c
@@ -484,7 +484,14 @@ static void __init init_pdx(void)
 {
     paddr_t bank_start, bank_size, bank_end;
 
-    uint64_t mask = pdx_init_mask(bootinfo.mem.bank[0].start);
+    /*
+     * Arm does not have any restrictions on the bits to compress. Pass 0 to
+     * let the common code further restrict the mask.
+     *
+     * If the logic changes in pfn_pdx_hole_setup we might have to
+     * update this function too.
+     */
+    uint64_t 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 8356f03ce8..c91875fabe 100644
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -50,9 +50,11 @@ static u64 __init fill_mask(u64 mask)
     return mask;
 }
 
+/* We don't want to compress the low MAX_ORDER bits of the addresses. */
 uint64_t __init pdx_init_mask(uint64_t base_addr)
 {
-    return fill_mask(base_addr - 1);
+    return fill_mask(max(base_addr,
+                         (uint64_t)1 << (MAX_ORDER + PAGE_SHIFT)) - 1);
 }
 
 u64 __init pdx_region_mask(u64 base, u64 len)
@@ -80,6 +82,9 @@ 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 the ARM specific
+     * init_pdx too.
      */
     for ( j = MAX_ORDER-1; ; )
     {
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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