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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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).

> 
> > 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,
> 



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel