Re: [Xen-devel] [PATCH 2/4] xen/arm: Implement a dummy Performance Monitor for ARM32

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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
> Monitor registers") disable Performance Monitor.
> 
> When CONFIG_PERF_EVENTS is enabled in the Linux Kernel, regardless the
> ID_DFR0 (which tell if Perfomance Monitors Extension is implemented) the
> kernel will try to access to PMCR.
> 
> Therefore we tell the guest we have 0 counters. Unfortunately we must always
> support PMCCNTR (the cycle counter): we just RAZ/WI for all PM register,
> which doesn't crash the kernel at least.

How often does this trap occur in practice? Once at start of day? Only
if you run perf? Or on every guest context switch? (obviously the last
one would be bad...)



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