Re: [Xen-devel] ARM - why does setup_frametable_size() round frametable_size to 32MB ?

["Chris (Christopher) Brand" <chris.brand@xxxxxxxxxxxx>]

Thanks, Ian. I tried that, and it does seem to work (everything boots, I can 
still bring up VMs, and I see an extra 16MB of free memory). The patch I came 
up with follows (it would be nice to share code between create_32mb_mappings() 
and create_2mb_mappings(), but the setting of the contig bit is right in the 
middle, and the functions are pretty short).

Chris

From: Chris Brand <chris.brand@xxxxxxxxxxxx>
Date: Mon, 20 Jul 2015 13:38:15 -0700
Subject: [PATCH] xen: arm: Support <32MB frametables

setup_frametable_mappings() rounds frametable_size up to a multiple
of 32MB. This is wasteful on systemes with less than 4GB of RAM,
although it does allow the "contig" bit to be set in the PTEs.

Where the frametable is less than 32MB in size, instead round up
to a multiple of 2MB, not setting the "contig" bit in the PTEs.

Signed-off-by: Chris Brand <chris.brand@xxxxxxxxxxxx>
---
 xen/arch/arm/mm.c | 39 ++++++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index a91ea774f1f9..a7f4864f8d8f 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -656,6 +656,29 @@ static void __init create_32mb_mappings(lpae_t *second,
 }
 
 #ifdef CONFIG_ARM_32
+static void __init create_2mb_mappings(lpae_t *second,
+                                       unsigned long virt_offset,
+                                       unsigned long base_mfn,
+                                       unsigned long nr_mfns)
+{
+    unsigned long i, count;
+    lpae_t pte, *p;
+
+    ASSERT(!((virt_offset >> PAGE_SHIFT) % LPAE_ENTRIES));
+    ASSERT(!(base_mfn % LPAE_ENTRIES));
+    ASSERT(!(nr_mfns % LPAE_ENTRIES));
+
+    count = nr_mfns / LPAE_ENTRIES;
+    p = second + second_linear_offset(virt_offset);
+    pte = mfn_to_xen_entry(base_mfn, WRITEALLOC);
+    for ( i = 0; i < count; i++ )
+    {
+        write_pte(p + i, pte);
+        pte.pt.base += 1 << LPAE_SHIFT;
+    }
+    flush_xen_data_tlb_local();
+}
+
 /* Set up the xenheap: up to 1GB of contiguous, always-mapped memory. */
 void __init setup_xenheap_mappings(unsigned long base_mfn,
                                    unsigned long nr_mfns)
@@ -749,6 +772,7 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t 
pe)
     unsigned long nr_pdxs = pfn_to_pdx(nr_pages);
     unsigned long frametable_size = nr_pdxs * sizeof(struct page_info);
     unsigned long base_mfn;
+    unsigned long mask;
 #ifdef CONFIG_ARM_64
     lpae_t *second, pte;
     unsigned long nr_second, second_base;
@@ -757,8 +781,12 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t 
pe)
 
     frametable_base_pdx = pfn_to_pdx(ps >> PAGE_SHIFT);
 
-    /* Round up to 32M boundary */
-    frametable_size = (frametable_size + 0x1ffffff) & ~0x1ffffff;
+    /* Round up to 2M or 32M boundary, as appropriate */
+    if (frametable_size < MB(32))
+        mask = MB(2) - 1;
+    else
+        mask = MB(32) - 1;
+    frametable_size = (frametable_size + mask) & ~mask;
     base_mfn = alloc_boot_pages(frametable_size >> PAGE_SHIFT, 32<<(20-12));
 
 #ifdef CONFIG_ARM_64
@@ -773,7 +801,12 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t 
pe)
     }
     create_32mb_mappings(second, 0, base_mfn, frametable_size >> PAGE_SHIFT);
 #else
-    create_32mb_mappings(xen_second, FRAMETABLE_VIRT_START, base_mfn, 
frametable_size >> PAGE_SHIFT);
+    if (frametable_size < MB(32))
+        create_2mb_mappings(xen_second, FRAMETABLE_VIRT_START,
+                            base_mfn, frametable_size >> PAGE_SHIFT);
+    else
+        create_32mb_mappings(xen_second, FRAMETABLE_VIRT_START,
+                             base_mfn, frametable_size >> PAGE_SHIFT);
 #endif
 
     memset(&frame_table[0], 0, nr_pdxs * sizeof(struct page_info));
-- 
1.9.1



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