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

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Message-ID: <781a2f80-9ca6-4875-9b4a-ecef7694ae2e@suse.com>
Date: Thu, 19 Dec 2024 08:12:18 +0100
From: Jan Beulich <jbeulich@...e.com>
To: Jürgen Groß <jgross@...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 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.

Jan