From: Julien Grall <jgrall@amazon.com>

The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st slot.

Add some BUILD_BUG_ON() to confirm that. This is helpful if one wants
to re-order the memory layout.

Signed-off-by: Julien Grall <jgrall@amazon.com>
---
 xen/arch/arm/mm.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 8a17222109c6..40423a70f0ae 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -211,6 +211,22 @@ static void __init __maybe_unused build_assertions(void)
 #ifdef CONFIG_DOMAIN_PAGE
     BUILD_BUG_ON(DOMHEAP_VIRT_START & ~FIRST_MASK);
 #endif
+    /*
+     * The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
+     * BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st
+     * slot.
+     */
+#define CHECK_SAME_SLOT(level, virt1, virt2) \
+    BUILD_BUG_ON(level##_table_offset(virt1) != level##_table_offset(virt2))
+
+#ifdef CONFIG_ARM_64
+    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, FIXMAP_ADDR(0));
+    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, BOOT_FDT_VIRT_START);
+#endif
+    CHECK_SAME_SLOT(first, XEN_VIRT_START, FIXMAP_ADDR(0));
+    CHECK_SAME_SLOT(first, XEN_VIRT_START, BOOT_FDT_VIRT_START);
+
+#undef CHECK_SAME_SLOT
 }
 
 void dump_pt_walk(paddr_t ttbr, paddr_t addr,
-- 
2.32.0

Hi Julien,

> On 28 Feb 2022, at 10:06, Julien Grall <julien@xen.org> wrote:
> 
> From: Julien Grall <jgrall@amazon.com>
> 
> The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
> BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st slot.
> 
> Add some BUILD_BUG_ON() to confirm that. This is helpful if one wants
> to re-order the memory layout.

Very good idea :-)

> 
> Signed-off-by: Julien Grall <jgrall@amazon.com>

Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

Just a small NIT after if you want to do it on commit...

> ---
> xen/arch/arm/mm.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 8a17222109c6..40423a70f0ae 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -211,6 +211,22 @@ static void __init __maybe_unused build_assertions(void)
> #ifdef CONFIG_DOMAIN_PAGE
>     BUILD_BUG_ON(DOMHEAP_VIRT_START & ~FIRST_MASK);
> #endif
> +    /*
> +     * The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
> +     * BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st
> +     * slot.

NIT: I would just add at the end of the sentence “in the page tables.”

Cheers
Bertrand

> +     */
> +#define CHECK_SAME_SLOT(level, virt1, virt2) \
> +    BUILD_BUG_ON(level##_table_offset(virt1) != level##_table_offset(virt2))
> +
> +#ifdef CONFIG_ARM_64
> +    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, FIXMAP_ADDR(0));
> +    CHECK_SAME_SLOT(zeroeth, XEN_VIRT_START, BOOT_FDT_VIRT_START);
> +#endif
> +    CHECK_SAME_SLOT(first, XEN_VIRT_START, FIXMAP_ADDR(0));
> +    CHECK_SAME_SLOT(first, XEN_VIRT_START, BOOT_FDT_VIRT_START);
> +
> +#undef CHECK_SAME_SLOT
> }
> 
> void dump_pt_walk(paddr_t ttbr, paddr_t addr,
> -- 
> 2.32.0
> 

On 03/03/2022 16:04, Bertrand Marquis wrote:
> Hi Julien,

Hi Bertrand,

>> On 28 Feb 2022, at 10:06, Julien Grall <julien@xen.org> wrote:
>>
>> From: Julien Grall <jgrall@amazon.com>
>>
>> The boot code expects the regions XEN_VIRT_START, FIXMAP_ADDR(0),
>> BOOT_FDT_VIRT_START to use the same 0th (arm64 only) and 1st slot.
>>
>> Add some BUILD_BUG_ON() to confirm that. This is helpful if one wants
>> to re-order the memory layout.
> 
> Very good idea :-)

I actually hit this issue when trying to re-order the memory layout for 
testing a patch.

> 
>>
>> Signed-off-by: Julien Grall <jgrall@amazon.com>
> 
> Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
> 
> Just a small NIT after if you want to do it on commit...

I have update the comment and committed the patch.

Thanks!

Cheers,

-- 
Julien Grall