Re: [PATCH 1/2] arm64: PMU: Do not use PMSELR_EL0 to access PMCCFILTR_EL0

[Marc Zyngier <marc.zyngier@arm.com>]

On 06/12/16 13:50, Will Deacon wrote:
> On Fri, Dec 02, 2016 at 03:50:58PM +0000, Marc Zyngier wrote:
>> The ARMv8 architecture allows the cycle counter to be configured
>> by setting PMSELR_EL0.SEL==0x1f and then accessing PMXEVTYPER_EL0,
>> hence accessing PMCCFILTR_EL0. But it disallows the use of
>> PMSELR_EL0.SEL==0x1f to access the cycle counter itself through
>> PMXEVCNTR_EL0.
>>
>> Linux itself doesn't violate this rule, but we may end up with
>> PMSELR_EL0.SEL being set to 0x1f when we enter a guest. If that
>> guest accesses PMXEVCNTR_EL0, the access may UNDEF at EL1,
>> despite the guest not having done anything wrong.
>>
>> In order to avoid this unfortunate course of events (haha!), let's
>> apply the same method armv8pmu_write_counter and co are using,
>> explicitely checking for the cycle counter and writing to
>> PMCCFILTR_EL0 directly. This prevents writing 0x1f to PMSELR_EL0,
>> and saves a Linux guest an extra trap.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  arch/arm64/kernel/perf_event.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
>> index 57ae9d9..a65b757 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -632,7 +632,10 @@ static inline void armv8pmu_write_counter(struct perf_event *event, u32 value)
>>  
>>  static inline void armv8pmu_write_evtype(int idx, u32 val)
>>  {
>> -	if (armv8pmu_select_counter(idx) == idx) {
>> +	if (idx == ARMV8_IDX_CYCLE_COUNTER) {
>> +		val &= ARMV8_PMU_EVTYPE_MASK & ~ARMV8_PMU_EVTYPE_EVENT;
>> +		write_sysreg(val, pmccfiltr_el0);
>> +	} else if (armv8pmu_select_counter(idx) == idx) {
> 
> If we go down this route, then we also have to "fix" the 32-bit code,
> which uses PMSELR in a similar way. However, neither of the perf drivers
> are actually doing anything wrong here -- the problem comes about because
> the architecture doesn't guarantee that PMU accesses trap to EL2 unless
> both MDCR.TPM=1 *and* PMSELR_EL0 is valid. So I think that this should
> be handled together, in the KVM code that enables PMU traps.
> 
> Given that the perf callbacks tend to run with preemption disabled, I
> think you should be fine nuking PMSELR_EL0 to zero (i.e. no need to
> save/restore).

Fair enough. I'll respin another patch in a bit.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...