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

[Juergen Gross <jgross@suse.com>]

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On 12.12.24 11:22, Jan Beulich wrote:
> On 11.12.2024 19:20, Thierry Escande wrote:
>> Hi Jan,
>>
>> On 09/12/2024 11:04, Jan Beulich wrote:
>>> 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.
>>
>> Thanks for pointing this out. On the Xen side, CONFIG_CTLDOM_MAX_ORDER
>> and CONFIG_HWDOM_MAX_ORDER seem big enough on all architectures. But I
>> see CONFIG_DOMU_MAX_ORDER set to 9 (also all archs). Won't that be a
>> problem for drivers trying to allocate more than that from a domU ?
> 
> A driver assumes a (physical) device to be in the DomU, at which point it
> is CONFIG_PTDOM_MAX_ORDER which applies (PT standing for pass-through).
> 
>>> 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.
>>
>> Since this works on bare metal I'm afraid the driver maintainer (mpt
>> fusion driver) will just tell me to fix Xen.
> 
> Well. The bigger such allocations, the larger the risk that on systems
> that have been up for a while such allocations can't be fulfilled anymore
> even in the bare metal case.

Yes. I don't think we should just work around this issue without having
even tried to get the driver fixed. In case they refuse to change it, we
can still increase MAX_CONTIG_ORDER.


Juergen

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