[Xen-devel] [PATCH v3] xen: arm: rework placement of fdt in initial dom0 memory map

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

The 32-bit Linux kernel uses its lowmem direct mapping to access the FDT. The
lowmem mapping is around 0.75GiB but varies depending on the kernel's .config.
Our current scheme of loading the FDT as high as 4GB therefore fails with
larger amounts of dom0 RAM.

The upstream documentation has recently been update to provide more guidance
<http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=7824/1>. In
accordance with this load the kernel just below 128MiB (aligned to 2MB) and
the FDT just above, or if there is less RAM available then as high as
possible.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
---
v3: Improve checking on final location of dtb, to make sure it is in guest RAM
    Use new_size for dtbsize where possible
    Remove pointless subtractions of start when it is always zero
v2: Use the kernels own reported size rather than the size of the module. This
    is consistent with the amount we will actually copy (zimage.len)
v1: This is technically v2 of "xen: arm: load FDT below 0.5G"
---
 xen/arch/arm/domain_build.c | 23 +++++++++++++++--------
 xen/arch/arm/kernel.c       | 27 ++++++++++++++++++++++++---
 2 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
index 77e2a61..34d3320 100644
--- a/xen/arch/arm/domain_build.c
+++ b/xen/arch/arm/domain_build.c
@@ -503,19 +503,27 @@ static int prepare_dtb(struct domain *d, struct 
kernel_info *kinfo)
     if ( ret < 0 )
         goto err;
 
+    /* Actual new size */
+    new_size = fdt_totalsize(kinfo->fdt);
+
     /*
-     * DTB must be load below 4GiB and far enough from linux (Linux uses
-     * the space after it to decompress)
-     * Load the DTB at the end of the first bank, while ensuring it is
-     * also below 4G
+     * DTB must be loaded such that it does not conflict with the
+     * kernel decompressor. For 32-bit Linux Documentation/arm/Booting
+     * recommends just after the 128MB boundary while for 64-bit Linux
+     * the recommendation in Documentation/arm64/booting.txt is below
+     * 512MB. Place at 128MB, (or, if we have less RAM, as high as
+     * possible) in order to satisfy both.
      */
     end = kinfo->mem.bank[0].start + kinfo->mem.bank[0].size;
-    end = MIN(1ull << 32, end);
-    kinfo->dtb_paddr = end - fdt_totalsize(kinfo->fdt);
+    end = MIN(kinfo->mem.bank[0].start + (128<<20) + new_size, end);
+
+    kinfo->dtb_paddr = end - new_size;
+
     /* Align the address to 2Mb. Linux only requires 4 byte alignment */
     kinfo->dtb_paddr &= ~((2 << 20) - 1);
 
-    if ( fdt_totalsize(kinfo->fdt) > end )
+    if ( kinfo->dtb_paddr < kinfo->mem.bank[0].start ||
+         kinfo->mem.bank[0].start + new_size > end )
     {
         printk(XENLOG_ERR "Not enough memory in the first bank for "
                "the device tree.");
@@ -523,7 +531,6 @@ static int prepare_dtb(struct domain *d, struct kernel_info 
*kinfo)
         goto err;
     }
 
-
     return 0;
 
   err:
diff --git a/xen/arch/arm/kernel.c b/xen/arch/arm/kernel.c
index f12f895..7a91d60 100644
--- a/xen/arch/arm/kernel.c
+++ b/xen/arch/arm/kernel.c
@@ -211,11 +211,32 @@ static int kernel_try_zimage32_prepare(struct kernel_info 
*info,
     info->zimage.kernel_addr = addr;
 
     /*
-     * If start is zero, the zImage is position independent -- load it
-     * at 32k from start of RAM.
+     * If start is zero, the zImage is position independent, in this
+     * case Documentation/arm/Booting recommends loading below 128MiB
+     * and above 32MiB. Load it as high as possible within these
+     * constraints, while also avoiding the DTB.
      */
     if (start == 0)
-        info->zimage.load_addr = info->mem.bank[0].start + 0x8000;
+    {
+        paddr_t load_end;
+
+        load_end = info->mem.bank[0].start + info->mem.bank[0].size;
+        load_end = MIN(info->mem.bank[0].start + (128<<20), load_end);
+
+        /*
+         * FDT is loaded above 128M or as high as possible, so the
+         * only way we can clash is if we have <=128MB, in which case
+         * FDT will be right at the end and so dtb_paddr will be below
+         * the proposed kernel load address. Move the kernel down if
+         * necessary.
+         */
+        if ( load_end >= info->dtb_paddr )
+            load_end = info->dtb_paddr;
+
+        info->zimage.load_addr = load_end - end;
+        /* Align to 2MB */
+        info->zimage.load_addr &= ~((2 << 20) - 1);
+    }
     else
         info->zimage.load_addr = start;
     info->zimage.len = end - start;
-- 
1.8.3.2


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