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

["Jürgen Groß" <jgross@suse.com>]

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On 18.12.24 12:11, Thierry Escande wrote:
> 
> 
> On 12/12/2024 12:09, Juergen Gross wrote:
>> 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.
> 
> Thanks for the feedback. I'll try to have a look at the driver if I have
> time to do so.

Another thought would be to change the generic DMA allocation to not require
alignment based on the rounded up size, but on the largest power-of-2 chunk
fitting into the requested size.

I don't see why a 2.3 MB memory allocation would need to be 4 MB aligned. It
should be perfectly fine to align it to 2 MB only.


Juergen

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