Re: [PATCH] xen/arm: increase default dom0_mem to 512M

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 15/02/2017 23:05, Stefano Stabellini wrote:
> The default dom0_mem is 128M which is not sufficient to boot a Ubuntu
> based Dom0. Increase it to 512M.
>
> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

I am not a big fan of increasing the default value. 128M is plenty 
enough if you use a small DOM0 (e.g buildroot or yocto) and people may 
rely on it because it is the default value in the documentation
(see docs/misc/xen-command-line.markdown).

Also, 512M may boot Ubuntu for you but it might not be the case in all 
the configuration. There is no perfect default value, but I think the 
smaller is better. Looking at the documentation, it looks like x86 is 
using 128MB or 1/16 of the memory (whichever is smaller).

But to be fair, I am not even sure why there is a default value, it is 
quite easy to specify the amount of memory used by DOM0 on the command line.

> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index c97a1f5..f4612a2 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -31,7 +31,7 @@ integer_param("dom0_max_vcpus", opt_dom0_max_vcpus);
>
>  int dom0_11_mapping = 1;
>
> -#define DOM0_MEM_DEFAULT 0x8000000 /* 128 MiB */
> +#define DOM0_MEM_DEFAULT 0x20000000 /* 512 MiB */

I would use the MB(..) macro here to make the code more readable.

>  static u64 __initdata dom0_mem = DOM0_MEM_DEFAULT;
>
>  static void __init parse_dom0_mem(const char *s)
>

Cheers,

-- 
Julien Grall

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