Re: [XEN v1] Xen: Enable compilation when PADDR_BITS == BITS_PER_LONG

[Julien Grall <julien@xxxxxxx>]

Hi Ayan,

On 01/12/2022 10:03, Ayan Kumar Halder wrote:
> It is possible for a pointer to represent physical memory of the same size.
> In other words, a 32 bit pointer can represent 32 bit addressable physical
> memory.
> Thus, issue a compilation failure only when the count of physical address bits
> is greater than BITS_PER_LONG (ie count of bits in void*).
I am having difficult to understand how this description is related to 
the BUILD_BUG_ON(). AFAIU, it is used to check that xenheap_bits can be 
used in shift.
If the unsigned long is 32-bit, then a shift of 32 could be undefined. 
Looking at the current use, the shift are used with "xenheap_bits - 
PAGE_SHIFT". So as long as PAGE_SHIFT is not 0, you would be fine.
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> ---
>
> Currently this change will not have any impact on the existing architectures.
> The following table illustrates PADDR_BITS vs BITS_PER_LONG of different archs
>
> ------------------------------------------------
> | Arch      |   PADDR_BITS    |   BITS_PER_LONG |
> ------------------------------------------------
> | Arm_64    |   48            |   64            |
> | Arm_32    |   40            |   32            |
> | RISCV_64  |   Don't know    |   64            |
> | x86       |   52            |   64            |
> -------------------------------------------------
The Arm_32 line is a bit confusing because one would wonder why we 
haven't seen this issue yet. So I think you want to clarify that the 
code path is not used by Arm32.
> However, this will change when we introduce a platform (For eg Cortex-R52) which
> supports 32 bit physical address and BITS_PER_LONG.
> Thus, I have introduced this change as I don't see it causing a regression on
> any of the supported platforms.
>
>   xen/common/page_alloc.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> index 62afb07bc6..cd390a0956 100644
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -2245,7 +2245,7 @@ void __init xenheap_max_mfn(unsigned long mfn)
>   {
>       ASSERT(!first_node_initialised);
>       ASSERT(!xenheap_bits);
> -    BUILD_BUG_ON(PADDR_BITS >= BITS_PER_LONG);
> +    BUILD_BUG_ON(PADDR_BITS > BITS_PER_LONG);
Based on the above, I think this wants to be "(PADDR_BITS - PAGE_SHIFT) 
>= BITS_PER_LONG)".
>       xenheap_bits = min(flsl(mfn + 1) - 1 + PAGE_SHIFT, PADDR_BITS);
>       printk(XENLOG_INFO "Xen heap: %u bits\n", xenheap_bits);
>   }
Cheers,

--
Julien Grall