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

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

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?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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