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[Xen-devel] [PATCH] xen: arm: configure correct dom0_gnttab_start/size

Vexpress is currently failing to boot for me with:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 1 at arch/arm/mm/ioremap.c:301 
CPU: 0 PID: 1 Comm: swapper Tainted: G        W     3.16.0-arm-native+ #276
[<c0011e9c>] (unwind_backtrace) from [<c0010758>] (show_stack+0x10/0x14)
[<c0010758>] (show_stack) from [<c001a3ec>] (warn_slowpath_common+0x5c/0x7c)
[<c001a3ec>] (warn_slowpath_common) from [<c001a4c8>] 
[<c001a4c8>] (warn_slowpath_null) from [<c001488c>] 
[<c001488c>] (__arm_ioremap_pfn_caller) from [<c00149a0>] 
[<c00149a0>] (__arm_ioremap) from [<c01d103c>] 
[<c01d103c>] (gnttab_setup_auto_xlat_frames) from [<c0495324>] 
[<c0495324>] (xen_guest_init) from [<c0492c6c>] (do_one_initcall+0xfc/0x1a4)
[<c0492c6c>] (do_one_initcall) from [<c0492d6c>] 
[<c0492d6c>] (kernel_init_freeable) from [<c039611c>] (kernel_init+0x8/0xe4)
[<c039611c>] (kernel_init) from [<c000de58>] (ret_from_fork+0x14/0x3c)
---[ end trace 3406ff24bd97382f ]---
xen:grant_table: Failed to ioremap gnttab share frames 

which is:
         * Don't allow RAM to be mapped - this causes problems with ARMv6+
        if (WARN_ON(pfn_valid(pfn)))
                return NULL;

This makes sense since the gnttab defaults to 0xb000000 and my dom0
is being allocated a 1:1 mapping at 0xa0000000-0xc0000000.

I suspect this broke around the time we stopped forcing dom0 memory to be
allocated as low as possible which happened to prevent the default dom0_gnttab
region overlapping RAM.

This patch specifies an explicit dom0_gnttab base which is explicitly unused
according to the FVP model docs (although it correpsonds to CS5 this isn't
wired up to anything).

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
This just fixes vexpress, I wonder if a followup patch should either remove the
default dom0_gnttab (forcing all platforms to specify one explicitly) or make
the default something less arbitrary than 0xb0000000, e.g. 0x0-0x20000 or
0xfffe0000-0x100000000 (very start or very end of RAM). Perhaps with a command
line option to override for new platform hacking.

Or maybe we should search for an unused hole in the dom0 RAM space?
 xen/arch/arm/platforms/vexpress.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/arm/platforms/vexpress.c 
index 8e6a4ea..ce66935 100644
--- a/xen/arch/arm/platforms/vexpress.c
+++ b/xen/arch/arm/platforms/vexpress.c
@@ -176,6 +176,8 @@ PLATFORM_START(vexpress, "VERSATILE EXPRESS")
     .reset = vexpress_reset,
     .blacklist_dev = vexpress_blacklist_dev,
+    .dom0_gnttab_start = 0x10000000,
+    .dom0_gnttab_size = 0x20000,

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