Re: [Xen-devel] [PATCH 4/5] xen: fix handling framebuffer located above 4GB

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, May 07, 2019 at 10:12:13AM -0600, Jan Beulich wrote:
> >>> On 07.05.19 at 17:38, <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > What do you think about adding something that could be backported?
> > Xen is quite insistent on initializing framebuffer, even with
> > console=com1 or console=none. Which means, there is no workaround for
> > this problem.
> 
> When the system is in a simple text mode the /basevideo option of
> xen.efi ought to help, but if it's in an LFB-based mode already (which
> is the case on the systems I have) then indeed I can't see any
> workaround.
> 
> > Maybe, as a first step, a change that abort framebuffer initialization
> > if lfb_base == 0 (I assume this is never valid value here, right?)?
> 
> Yes, that would be an option. But it would help only in your specific
> case, not if the truncation results in a non-zero value. I guess we'd
> better avoid filling the structure if we'd truncate the value.

Yes, I was thinking about setting lfb_base=0 explicitly if it would
overflow otherwise.

> But what's wrong with backporting your change as is?

If this commit would be backported, what value you'd put in that #ifdef?
Also, one may argue that ABI changes should not be backported... But
since there is clear and independent of xen version method of detecting
it, I don't think this would be a big issue here.

> > If not, then at least abort boot when text console is still there
> > (blexit before efi_exit_boot). Any preference?
> 
> What's wrong with the text console still active? Or maybe I'm
> misunderstandint you make...

As soon as you call ExitBootServices(), you can't use
SIMPLE_TEXT_OUTPUT_INTERFACE anymore. Which means if a) framebuffer
address didn't fit, and b) you called ExitBootServices() already, you
don't have any means to tell the user what is wrong. Other than serial
console of course, if you're lucky enough to have one. So the idea was
to report the problem before ExitBootServices().

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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