Re: [Minios-devel] [UNIKRAFT PATCHv5 20/46] plat/common: Add early debug console library for Arm64

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 11/09/18 06:35, Wei Chen (Arm Technology China) wrote:
> > > Oh, You beat me. Yes, PL011 start at IPA 0 is possible. But I don't know
> > > how to distinguish PL011 at IPA 0 or #ifndef
> > CONFIG_KVM_EARLY_DEBUG_PL011_UART.
> > > I had tried not to check (!pl011_uart_bas), it will generate an exception,
> > > and the exception entry will call PL011 to print message. It's an infinite
> > loop.
> >
> > If I understand correctly, KVM_EARLY_DEBUG_PL011_UART will exist if
> > KVM_DEBUG_SERIAL_CONSOLE is set. So one solution would be to introduce
> > an extra variable to check whether the UART has been initialized.
> >
> > This would be set to 1 at boot when KVM_DEBUG_SERIAL_CONSOLE is set.
> Ok, I understand now.
>
> Just a digression, if an IPA 0 is possible, so I think most of the NULL
> Check would be unreliable. For example, lots of fdt_get_property will
> return a pointer. You know, most of us will use the if(!pointer) to check
> the return value.
Well, that the drawback with building page-table with VA == PA. You have 
somehow to ensure that VA 0 is never mapped.
For now, you could just check whether the VA 0 has been mapped and print 
a warning. This would need to be properly fixed sooner or later as it 
looks like some hardware have PA 0 valid.
Cheers,

--
Julien Grall

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