Re: [PATCH for-4.16] xen/arm: don't assign domU static-mem to dom0 as reserved-memory

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 09/11/2021 00:48, Stefano Stabellini wrote:
> From: Stefano Stabellini <stefano.stabellini@xilinx.com>
> 
> DomUs static-mem ranges are added to the reserved_mem array for
> accounting, but they shouldn't be assigned to dom0 as the other regular
> reserved-memory ranges in device tree.
> 
> In make_memory_nodes, fix the error by skipping banks with xen_domain
> set to true in the reserved-memory array. Also make sure to use the
> first valid (!xen_domain) start address for the memory node name.
> 

This is a bug fix. So please add a Fixes tag. In this case, I think it 
should be:

Fixes: 41c031ff437b ("xen/arm: introduce domain on Static Allocation")

> Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
> ---
> 
> This patch should be considered for 4.16 as it fixes an incorrect
> behavior.
> 
> The risk is low for two reasons:
> - the change is simple
> - make_memory_node is easily tested because it gets called at every
>    boot, e.g. gitlab-ci and OSSTest exercise this path
> 
> I tested this patch successfully with and without xen,static-mem.
> 
> ---
>   xen/arch/arm/domain_build.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
> index 1fbafeaea8..56d3ff9d08 100644
> --- a/xen/arch/arm/domain_build.c
> +++ b/xen/arch/arm/domain_build.c
> @@ -874,11 +874,17 @@ static int __init make_memory_node(const struct domain *d,
>       if ( mem->nr_banks == 0 )
>           return -ENOENT;
>   
> +    for ( i = 0; i < mem->nr_banks && mem->bank[i].xen_domain; i++ )
> +        ;
> +    /* No reserved-memory ranges to expose to Dom0 */
I find this comment a bit misleading because make_memory_node() will 
also be called to create normal memory region. I would drop 
"reserved-memory" and add a comment on top of the loop explaining what 
the loop does.

> +    if ( i == mem->nr_banks )
> +        return 0;
> +
>       dt_dprintk("Create memory node (reg size %d, nr cells %d)\n",
>                  reg_size, nr_cells);

I think you need to adjust nr_cells otherwise we would write garbagge in 
the DT if we need to exclude some regions.

>   
>       /* ePAPR 3.4 */
> -    snprintf(buf, sizeof(buf), "memory@%"PRIx64, mem->bank[0].start);
> +    snprintf(buf, sizeof(buf), "memory@%"PRIx64, mem->bank[i].start);
>       res = fdt_begin_node(fdt, buf);
>       if ( res )
>           return res;
> @@ -888,11 +894,14 @@ static int __init make_memory_node(const struct domain *d,
>           return res;
>   
>       cells = &reg[0];
> -    for ( i = 0 ; i < mem->nr_banks; i++ )
> +    for ( ; i < mem->nr_banks; i++ )
>       {
>           u64 start = mem->bank[i].start;
>           u64 size = mem->bank[i].size;
>   
> +        if ( mem->bank[i].xen_domain )
> +            continue;
> +
>           dt_dprintk("  Bank %d: %#"PRIx64"->%#"PRIx64"\n",
>                      i, start, start + size);
>   
> 

Cheers,

-- 
Julien Grall