Re: [RFC v2 PATCH 7/7] arm64: KVM: Enable selective trapping of TLB instructions

[Punit Agrawal <punit.agrawal@arm.com>]

Christoffer Dall <christoffer.dall@linaro.org> writes:

> On Mon, Sep 05, 2016 at 05:31:37PM +0100, Punit Agrawal wrote:
>> The TTLB bit of Hypervisor Control Register (HCR_EL2) controls the
>> trapping of guest TLB maintenance instructions. Taking the trap requires
>> a switch to the hypervisor and is an expensive operation.
>> 
>> Enable selective trapping of guest TLB instructions when the associated
>> perf trace event is enabled for a specific virtual machine.
>> 
>> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  arch/arm64/kvm/perf_trace.c | 32 ++++++++++++++++++++++++++++++++
>>  1 file changed, 32 insertions(+)
>> 
>> diff --git a/arch/arm64/kvm/perf_trace.c b/arch/arm64/kvm/perf_trace.c
>> index 8bacd18..f26da1d 100644
>> --- a/arch/arm64/kvm/perf_trace.c
>> +++ b/arch/arm64/kvm/perf_trace.c
>> @@ -17,6 +17,8 @@
>>  #include <linux/kvm_host.h>
>>  #include <linux/trace_events.h>
>>  
>> +#include <asm/kvm_emulate.h>
>> +
>>  typedef int (*perf_trace_callback_fn)(struct kvm *kvm, bool enable);
>>  
>>  struct kvm_trace_hook {
>> @@ -24,7 +26,37 @@ struct kvm_trace_hook {
>>  	perf_trace_callback_fn setup_fn;
>>  };
>>  
>> +static int tlb_invalidate_trap(struct kvm *kvm, bool enable)
>> +{
>> +	int i;
>> +	struct kvm_vcpu *vcpu;
>> +
>> +	/*
>> +	 * Halt the VM to ensure atomic update across all vcpus (this
>> +	 * avoids racy behaviour against other modifications of
>> +	 * HCR_EL2 such as kvm_toggle_cache/kvm_set_way_flush).
>> +	 */
>> +	kvm_arm_halt_guest(kvm);
>> +	kvm_for_each_vcpu(i, vcpu, kvm) {
>> +		unsigned long hcr = vcpu_get_hcr(vcpu);
>> +
>> +		if (enable)
>> +			hcr |= HCR_TTLB;
>> +		else
>> +			hcr &= ~HCR_TTLB;
>> +
>> +		vcpu_set_hcr(vcpu, hcr);
>> +	}
>> +	kvm_arm_resume_guest(kvm);
>> +
>> +	return 0;
>> +}
>> +
>>  static struct kvm_trace_hook trace_hook[] = {
>> +	{
>> +		.key = "kvm_tlb_invalidate",
>
> is this key defined elsewhere?  If not, I think the name is ambiguous,
> because it's unclear if this means 'kvm the subsystem' or 'a kvm guest',
> by looking purely at the string.

The key is the name of the tracepoint that the callback is executed
against. Here it is the tracepoint defined in Patch 6/7.

I've now added a comment to the definition of "struct kvm_trace_hook" to
explain what the key is.

>
>> +		.setup_fn = tlb_invalidate_trap,
>> +	},
>>  	{ },
>>  };
>>  
>> -- 
>> 2.8.1
>> 
>
> Otherwise looks ok to me.
>
> -Christoffer
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