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

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Message-ID: <28947d4f-ab32-4a57-8dbb-e37fa4183a69@suse.com>
Date: Sat, 4 Jan 2025 19:50:03 +0100
From: Jürgen Groß <jgross@...e.com>
To: Jan Beulich <jbeulich@...e.com>
Cc: sstabellini@...nel.org, oleksandr_tyshchenko@...m.com,
 xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 Thierry Escande <thierry.escande@...es.tech>
Subject: Re: [PATCH] x86/xen/mmu: Increase MAX_CONTIG_ORDER

On 19.12.24 08:12, Jan Beulich wrote:
> On 18.12.2024 12:24, Jürgen Groß wrote:
>> 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@...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.
>>
>> 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.
> 
> Yet that wouldn't make a difference here, would it? We'd still need a 4M
> chunk of contiguous space, just with less alignment.

Thierry stated that the driver failed to load due to the added alignment
check introduced with commit 9f40ec84a797. I was targeting this reasoning
with my remark.


Juergen

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