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

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

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".

> I think arm64 makes more use than arm32 (unconditionally touches
> MDSCR_EL1 on the ctx switch path).
> 
> I think we should be considering allow the guest to access these and
> context switching them instead.

Disabling HW breakpoint don't disable debug. Linux will only use
software breakpoing (which is of course a bit slower).

I wrote this series to allow Distribution kernel (such as Linaro Ubuntu
kernel) boots correctly on Xen 4.4 and onwards.

I don't plan to more spend time to write a correct emulation (i.e
context switching) to support HW debug.

Regards,

-- 
Julien Grall