Re: Should KVM_GUEST stop depending on PARAVIRT?

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 28, 2015 at 05:23:22PM -0700, Andy Lutomirski wrote:
> 
> PeterZ, can we fix this for real instead of relying on
> CONFIG_PARAVIRT=y accidentally turning all msr accesses into "safe"
> accesses?  We have the CPUID "hypervisor" bit, but I still don't fully
> understand the problem.

So a whole bunch of PMU drivers already probe the MSRs at init time
using a combination of rdmsrl_safe() and wrmsrl_safe() to see if:

 1) its there at all
 2) if its there, it 'works' like it ought to

See for example: arch/x86/kernel/cpu/perf_event.c:check_hw_exists().

I have no problem using the _safe() methods to probe MSRs in init
routines and failing the init of the driver etc.

What I do object to is sprinkling _safe() all over the driver itself and
creating horrid error paths all over (yes, people have proposed crazy
things like that).

Another example is the LBR probing in
arch/x86/kernel/cpu/perf_event_intel.c:intel_pmu_init(). This is the one
people wanted to use _safe() crud for all throughout the driver, instead
of simply disabling the LBR at init time.


As to the 'bug' at hand, I really don't see the problem. The RAPL driver
says there's no valid RAPL domains, this is true, there aren't any on
the virtual machine, so what?