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

[Julien Grall <julien.grall@linaro.org>]

On 05/02/2014 02:14 PM, Ian Campbell wrote:
> On Fri, 2014-05-02 at 13:53 +0100, Julien Grall wrote:
>> On 05/02/2014 12:09 PM, Ian Campbell wrote:
>>> On Thu, 2014-04-24 at 23:45 +0100, Julien Grall wrote:
>>>> XSA-93 (commit 0b18220 "xen/arm: Don't let guess access to Debug and Performance
>>>> Monitors registers") disable Debug Registers access.
>>>>
>>>> When CONFIG_PERF_EVENTS is enabled in the Linux Kernel, it will try to
>>>> initialize the debug monitors. If an error occured Linux won't use this
>>>> feature.
>>>>
>>>> The implementation made Xen expose a minimal set of registers which let think
>>>> the guest (i.e.) thinks HW debug won't work.
>>>
>>> Why only for arm32?
>>
>> Because, if I'm not mistaken, you've already implemented a dummy HW
>> debug for arm64 in commit 0b182202 "xen/arm: Don't let guess access to
>> Debug and Performance Monitor registers".
> 
> That's a RAZ/WI thing, I thought this was something cleverer (returing
> values to make the guest think nothing was there).

Most of the time RAZ/WI is enough.  The arm32 implementation makes the
life harder.

I didn't check the arm64 implementation as I don't use it every day... I
need to set up the Foundation Model on my computer.

-- 
Julien Grall