Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG_ORDER

[Jan Beulich <jbeulich@suse.com>]

On 04.12.2024 18:14, Thierry Escande wrote:
> With change 9f40ec84a797 (xen/swiotlb: add alignment check for dma
> buffers), the driver mpt3sas fails to load because it cannot allocate
> its DMA pool for an allocation size of ~2,3 MBytes. This is because the
> alignement check added by 9f40ec84a797 fails and
> xen_swiotlb_alloc_coherent() ends up calling
> xen_create_contiguous_region() with a size order of 10 which is too high
> for the current max value.
> 
> This patch increases the MAX_CONTIG_ORDER from 9 to 10 (4MB) to allow
> such allocations.
> 
> Signed-off-by: Thierry Escande <thierry.escande@vates.tech>
> ---
>  arch/x86/xen/mmu_pv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 55a4996d0c04..7f110740e1a2 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -2200,7 +2200,7 @@ void __init xen_init_mmu_ops(void)
>  }
>  
>  /* Protected by xen_reservation_lock. */
> -#define MAX_CONTIG_ORDER 9 /* 2MB */
> +#define MAX_CONTIG_ORDER 10 /* 4MB */
>  static unsigned long discontig_frames[1<<MAX_CONTIG_ORDER];

While lacking respective commentary, bumping this value imo also needs to
take into account Xen itself, at least commit-message-wise. The bumping is
fine for Dom0 in any event. It is also fine for DomU-s with the defaults
built into the hypervisor (orders 12 and 10 respectively for x86 and Arm),
yet especially for Arm (and in the future PPC and RISC-V) any further
bumping would be less straightforward.

However - does the driver really need this big a contiguous chunk? It
would seem far more desirable to me to break that up some, if possible.

Jan