Re: [PATCH v3 01/10] drivers/char: Add support for USB3 DbC debugger

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.08.2022 16:28, Marek Marczykowski-Górecki wrote:
> On Thu, Aug 04, 2022 at 04:21:01PM +0200, Jan Beulich wrote:
>> On 04.08.2022 15:43, Marek Marczykowski-Górecki wrote:
>>> I need to keep this structure somewhere DMA-reachable for the device (as
>>> in - included in appropriate IOMMU context). Patch 8/10 is doing it. And
>>> also, patch 8/10 is putting it together with other DMA-reachable
>>> structures (not a separate page on its own). If I'd make it a separate
>>> static variable (not part of that later struct), I'd need to reserve the
>>> whole page for it - to guarantee no unrelated data lives on the same
>>> (DMA-reachable) page.
>>>
>>> As for statically initializing it, if would require the whole
>>> (multi-page DMA-reachable) thing living in .data, not .bss, so a bigger
>>> binary (not a huge concern due to compression, but still). But more
>>> importantly, I don't know how to do it in a readable way, and you have
>>> complained about readability of initializer of this structure in v2.
>>>
>>>> That struct will be quite a bit less than a page's worth in size.
>>>
>>> See above - it cannot share page with unrelated Xen data.
>>
>> I have to admit that I'd see no issue if these lived side by side with
>> e.g. other string literals. The more that the device is supposed to be
>> exposed to Dom0 only anyway, and hence that'll be the only domain able
>> to get at that data.
> 
> Other string literals are fine. But for example `struct dbc` itself is
> not.
> See how it is combined with other data in patch 8.
> 
>>>> If you build the file with -fshort-wchar, you may even be able to
>>>> use easy to read string literals for the initializer.
>>>
>>> I can try, but I'm not exactly sure how to make readable UTF-16
>>> literals...
>>
>> L"Xen" looks sufficiently readable to me. We use this all over the
>> place in the EFI interfacing code.
> 
> Ok, I can try that. But given later adjustments, IIUC it will make the
> whole 50+ pages structure land in .data. Is that okay?

No. I was actually expecting the piece of data we're talking about here
to land in .rodata, and hence be re-usable at the same address for all
devices. Hence my reference to string literals.

Jan