Re: [PATCH RFC] arm64/mm: handle memmap kernel option

[Ard Biesheuvel]

On 27 February 2017 at 10:24, Yisheng Xie <xieyisheng1@xxxxxxxxxx> wrote:
> hi Mark,
>
> Thanks for comment
> On 2017/2/27 16:48, Mark Rutland wrote:
>> Hi,
>>
>> On Mon, Feb 27, 2017 at 11:48:50AM +0800, Yisheng Xie wrote:
>>> On 2017/2/26 18:46, Ard Biesheuvel wrote:
>>>> On 25 February 2017 at 06:47, Yisheng Xie <xieyisheng1@xxxxxxxxxx> wrote:
>>
>>>>> To make user reserve memory for some specific use more convenient,
>>>>> this patch implement the following memmap variants:
>>>>>  - memmap=nn[KMG]$ss[KMG]: mark specified memory as reserved;
>>>>>  - memmap=nn[KMG]@ss[KMG]: force usage of a specific region of memory;
>>
>>>> Could you explain which problem you are solving here? ACPI implies
>>>> UEFI on arm64, and so these reservations could be made by a boot
>>>> component instead, if it requires a fixed memory reservation. If this
>>>> is a reservation for, e.g., OP-TEE, we should not rely on the command
>>>> line to communicate this information.
>>>>
>>> We just want to reserve some memory for a driver and I just not so familiar
>>> with how to reserve memory with UEFI. So doubt about whether it is suitable
>>> to reserve memory with cmdline like "memmap=xxx", which had appeared in x86
>>> for a long time.
>>
>> Could you please explain for what purpose this is necessary?
>>
>> Does the driver need a specific region of memory? Or just some contiguous
>> region? Or something else?
>>
> Yes, we want to use a specific region of memory.
>
>> For the former, this is not an appropriate solution; firmware must absolutely
>> mark the memory as reserved for a particular purpose.
>>

Indeed. The reason is that UEFI performs memory allocations itself,
and so putting memmap=xxx on the command line does not prevent the
region from being occupied when the kernel boots.

> I see, so just forget about this patch. sorry for disturbing.
>

No worries.

Thanks for the patch,
Ard.