Re: [PATCH v1 2/5] KVM: arm/arm64: Adjust entry/exit and trap related tracepoints

[Zenghui Yu]

Hi James,

On 2019/6/12 20:49, James Morse wrote:
> Hi,
>
> On 12/06/2019 10:08, Zenghui Yu wrote:
> > Currently, we use trace_kvm_exit() to report exception type (e.g.,
> > "IRQ", "TRAP") and exception class (ESR_ELx's bit[31:26]) together.
>
> (They both caused an exit!)
>
>
> > But hardware only saves the exit class to ESR_ELx on synchronous
>
> EC is the 'Exception Class'. Exit is KVM/Linux's terminology.
Yes, a stupid mistake ;-)

> > exceptions, not on asynchronous exceptions. When the guest exits
> > due to external interrupts, we will get tracing output like:
> >
> > 	"kvm_exit: IRQ: HSR_EC: 0x0000 (UNKNOWN), PC: 0xffff87259e30"
> >
> > Obviously, "HSR_EC" here is meaningless.
>
> I assume we do it this way so there is only one guest-exit tracepoint that catches all exits.
> I don't think its a problem if user-space has to know the EC isn't set for asynchronous
> exceptions, this is a property of the architecture and anything using these trace-points
> is already arch specific.
Actually, *no* problem in current implementation, and I'm OK to still
keep the EC in trace_kvm_exit().  What I really want to do is adding the
EC in trace_trap_enter (the new tracepoint), will explain it later.

> > This patch splits "exit" and "trap" events by adding two tracepoints
> > explicitly in handle_trap_exceptions(). Let trace_kvm_exit() report VM
> > exit events, and trace_kvm_trap_exit() report VM trap events.
> >
> > These tracepoints are adjusted also in preparation for supporting
> > 'perf kvm stat' on arm64.
>
> Because the existing tracepoints are ABI, I don't think we can change them.
>
> We can add new ones if there is something that a user reasonably needs to trace, and can't
> be done any other way.
>
> What can't 'perf kvm stat' do with the existing trace points?
(A good question! I should have made it clear in the commit message,
 forgive me.)

First, how does 'perf kvm stat' interact with tracepoints?

We have three handlers for a specific event (e.g., "VM-EXIT") --
"is_begin_event", "is_end_event", "decode_key". The first two handlers
make use of two existing tracepoints ("kvm:kvm_exit" & "kvm:kvm_entry")
to check when the VM-EXIT events started/ended, thus the time difference
stats, event start/end time etc. can be calculated.
"is_begin_event" handler gets a *key* from the "ret" field (exit_code)
of "kvm:kvm_exit" payload, and "decode_key" handler makes use of the
*key* to find out the reason for the VM-EXIT event. Of course we should
maintain the mapping between exit_code and exit_reason in userspace.
These are all what *patch #4* had done, #4 is a simple patch to review!
Oh, we can also set "vcpu_id_str" to achieve per vcpu event record, but
currently, we only have the "vcpu_pc" field in "kvm:kvm_entry", without
something like "vcpu_id".

perf people must have a much deeper understanding of this.


OK, next comes the more important question - what should/can we do to
the tracepoints in preparation of 'perf kvm stat' on arm64?

From the article you've provided, it's clear that we can't remove the EC
from trace_kvm_exit(). But can we add something like "vcpu_id" into
(at least) trace_kvm_entry(), just like what this patch has done?
If not, which means we have to keep the existing tracepoints totally
unchanged, then 'perf kvm stat' will have no way to record/report per
vcpu VM-EXIT events (other arch like X86, powerpc, s390 etc. have this
capability, if I understand it correctly).

As for TRAP events, should we consider adding two new tracepoints --
"kvm_trap_enter" and "kvm_trap_exit", to keep tracking of the trap
handling process? We should also record the EC in "kvm_trap_enter", 
which will be used as *key* in TRAP event's "is_begin_event" handler.
Patch #5 tells us the whole story, it's simple too.

What do you suggest?

> > diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> > index 516aead..af3c732 100644
> > --- a/arch/arm64/kvm/handle_exit.c
> > +++ b/arch/arm64/kvm/handle_exit.c
> > @@ -264,7 +264,10 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >   		exit_handle_fn exit_handler;
> >   
> >   		exit_handler = kvm_get_exit_handler(vcpu);
> > +		trace_kvm_trap_enter(vcpu->vcpu_id,
> > +				     kvm_vcpu_trap_get_class(vcpu));
> >   		handled = exit_handler(vcpu, run);
> > +		trace_kvm_trap_exit(vcpu->vcpu_id);
> >   	}
>
> Why are there two? Are you using this to benchmark the exit_handler()?
Almostly yes. Let perf know when the TRAP handling event start/end,
and ...

> As we can't remove the EC from the exit event, I don't think this tells us anything new.
As explained above, this EC is for 'perf kvm stat'.

> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index 90cedeb..9f63fd9 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -758,7 +758,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >   		/**************************************************************
> >   		 * Enter the guest
> >   		 */
> > -		trace_kvm_entry(*vcpu_pc(vcpu));
> > +		trace_kvm_entry(vcpu->vcpu_id, *vcpu_pc(vcpu));
>
> Why do you need the PC? It was exported on exit.
> (its mostly junk for user-space anyway, you can't infer anything from it)
(I mainly wanted to add the "vcpu->vcpu_id" here.)
It seems that we can't just remove the PC, which will cause ABI change?


Thanks for your reviewing!

zenghui


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