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

["Wei Chen (Arm Technology China)" <Wei.Chen@xxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien.grall@xxxxxxx>
> Sent: 2018年9月11日 18:37
> To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; Julien Grall
> <julien.grall@xxxxxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
> simon.kuenzer@xxxxxxxxx
> Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [Minios-devel] [UNIKRAFT PATCHv5 20/46] plat/common: Add early
> debug console library for Arm64
> 
> 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.
> 

Yes, at least I know that boot rom (flash) of QEMU/KVM is starting from IPA 0.
And we have already mapped it, because we have mapped 0 - 0x3FFFFFFF as device
MMIO. But I think only flash code need to care about the valid IPA 0, other code
can use NULL as normal. For Unikraft, I don’t think we would implement the
library for QEMU/KVM boot rom in recently.

Above comment is about current code for QEMU/KVM. I know some SoC would use PA0
for RAM, that would be more crazy : )

> Cheers,
> 
> --
> Julien Grall
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