Re: [Xen-devel] [PATCH 3/4] xen/arm: Implement a dummy debug monitor for ARM32

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 13/06/14 13:02, Ian Campbell wrote:
> Looking at it with fresh eyes this morning I see what you mean now, the
> essential difference is that with arm32 DBGDIDR is trapped by
> MDCR_EL2.TDA, whereas the the arm64 equivalent (IDAA64DFR*) are not
> (it's trapped as part of the ID register group, which we don't bother
> trapping). The rest of the registers are RAZ/WI which is consistent with
> arm64.
Right, this patch is mostly here to let Linux think there is not debug 
hardware registers.
As you said on another mail, the long term goal is to context switch 
correctly those register to allow perf and debug working in the guest.
> So in the end you've convinced me that this is the right thing to do for
> now and to backport to 4.4.
>
> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>
> I've committed this and the previous (perfc) one but not the next one
> (useful debug for coproc traps) which had comments.
>
> I did s/DBGCR/DBGBCR/ to match the name used in both the v7 and v8 ARM
> ARMs, I think it was just a typo? Hope that's ok.
Yes, I forgot the B by mistake.

> Also DBGOSLAR is supposed to be WO but you've implemented it as RAZ/WI,
> I didn't think that mattered enough to bother with though.
I was lazy to add 2 more lines to handle this register WO.

I'm not sure what is behavior when a guest is trying to read a WO 
register. I guess an undefined instruction.
I can send a follow-up to use this behavior for Xen 4.5.

Regards,

--
Julien Grall

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