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

Punit Agrawal
2016-09-06T11:34:09Z

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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