Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit width when platform is not set

[Julien Grall <julien@xen.org>]

On 20/08/2021 10:37, Wei Chen wrote:
> Hi Julien,

Hi Wei,

>> -----Original Message-----
>> From: Julien Grall <julien@xen.org>
>> Sent: 2021年8月20日 16:20
>> To: Wei Chen <Wei.Chen@arm.com>; xen-devel@lists.xenproject.org;
>> sstabellini@kernel.org; jbeulich@suse.com
>> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>
>> Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit width
>> when platform is not set
>>
>>
>>
>> On 20/08/2021 03:04, Wei Chen wrote:
>>> Hi Julien,
>>
>> Hi Wei,
>>
>>>> -----Original Message-----
>>>> From: Julien Grall <julien@xen.org>
>>>> Sent: 2021年8月19日 21:28
>>>> To: Wei Chen <Wei.Chen@arm.com>; xen-devel@lists.xenproject.org;
>>>> sstabellini@kernel.org; jbeulich@suse.com
>>>> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>
>>>> Subject: Re: [XEN RFC PATCH 04/40] xen/arm: return default DMA bit
>> width
>>>> when platform is not set
>>>>
>>>> Hi,
>>>>
>>>> On 11/08/2021 11:23, Wei Chen wrote:
>>>>> From: Hongda Deng <Hongda.Deng@arm.com>
>>>>>
>>>>> In current code, arch_get_dma_bitsize will return 32 when platorm
>>>>> or platform->dma_bitsize is not set. It's not resonable, for Arm,
>>>>
>>>> s/resonable/reasonable/
>>>>
>>>
>>> Ok
>>>
>>>>> we don't require to reserve DMA memory. So we set dma_bitsize always
>>>>> be 0. In NO-NUMA system, arch_get_dma_bitsize will not be invoked,
>>>>> so dma_bitsize will not be overrided by this function.
>>>>
>>>> arch_get_dma_bitsize() is also used to allocate dom0 memory. We need to
>>>> be able to allocate some DMA-able memory that can be used by every
>> devices.
>>>>
>>>>> But in NUMA
>>>>> system, once the online nodes are greater than 1, this function will
>>>>> be invoked. The dma_bitsize will be limited to 32. That means, only
>>>>> first 4GB memory can be used for DMA. But that's against our hardware
>>>>> design. We don't have that kind of restriction on hardware.
>>>>
>>>> What do you mean by "hardware design"? Are you referring to the server
>>>> you boot Xen on?
>>>>
>>>
>>> Yes. I will change it to some neutral words. something like:
>>> "But that could not reflect some hardware's real DMA ability. They may
>> not
>>> have kind of restriction on hardware." ?
>>
>> The thing is DMA ability is not about the platform itself. It is more
>> about the devices (this could just be a PCI card you just plugged). What
>> you seem to suggest is no-one will ever plug such card on your platform.
>> Is that correct?
>>
> 
> OK, I understand now. Let's keep 32-bit as default value, but even in this
> case, how about DMA-16 devices? Although these devices are very rare, they
> still exist : )

I haven't heard anyone reporting issues with them on Xen on Arm. So I 
assume that either it works or no-one is using them.

My main point is we need to care about the common use case. 32-bit DMA 
device is still a thing and caused trouble to some of our users (e.g. NXP).

If tomorrow, someone report issue with 16-bit DMA device, then we can 
consider our options how to handle.

>>>> So I would explore to remove the NUMA check for drop the DMA zone. FAOD,
>>>> both suggestion are for Arm only. For x86, they need to be kept.
>>>>
>>>
>>> Without introducing new flag, such as lowmem_for_dma, it's a little
>>> hard to skip the numa node check. Unless we crudely add #ifdef ARCH to
>>> common code, which is not what we want to see ...
>>>         if ( !dma_bitsize && (num_online_nodes() > 1) )
>>>             dma_bitsize = arch_get_dma_bitsize();
>>
>> ... Why do you think we need this check on Arm when NUMA is enabled?
>>
> 
> I didn't think Arm needs, what I said is introduce a flag to disable
> this check for Arm or other Architectures that they don't need this check.
> 
>> We can discuss how to remove it once this is answered.
>>
> 
> I think we can start to discuss it.

How about replacing the second part of the check with a new helper 
arch_have_default_dma_zone() (or a different name)?

Cheers,

-- 
Julien Grall