linux-kernel - Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG

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Message-Id: <ed764807-a58b-473c-911d-b52f013f89b2@vates.tech>
Date: Wed, 18 Dec 2024 11:11:45 +0000
From: "Thierry Escande" <thierry.escande@...es.tech>
To: "Juergen Gross" <jgross@...e.com>, "Jan Beulich" <jbeulich@...e.com>
Cc: sstabellini@...nel.org, oleksandr_tyshchenko@...m.com, xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG_ORDER



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@...es.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.

Regards,
Thierry