Re: [PATCH] trace "exit to userspace" event

[Gleb Natapov <gleb@redhat.com>]

On Sun, Oct 10, 2010 at 10:45:45AM +0200, Avi Kivity wrote:
>  On 10/08/2010 06:44 PM, Gleb Natapov wrote:
> >On Thu, Oct 07, 2010 at 09:49:17PM -0300, Marcelo Tosatti wrote:
> >>  On Thu, Oct 07, 2010 at 07:56:55PM +0200, Gleb Natapov wrote:
> >>  >  >  >  diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >>  >  >  >  index b8499f5..8800713 100644
> >>  >  >  >  --- a/virt/kvm/kvm_main.c
> >>  >  >  >  +++ b/virt/kvm/kvm_main.c
> >>  >  >  >  @@ -1458,6 +1458,8 @@ static long kvm_vcpu_ioctl(struct file *filp,
> >>  >  >  >   		if (arg)
> >>  >  >  >   			goto out;
> >>  >  >  >   		r = kvm_arch_vcpu_ioctl_run(vcpu, vcpu->run);
> >>  >  >  >  +		if (r>= 0)
> >>  >  >  >  +			trace_kvm_userspace_exit(vcpu->run->exit_reason);
> >>  >  >  >   		break;
> >>  >  >  >   	case KVM_GET_REGS: {
> >>  >  >  >   		struct kvm_regs *kvm_regs;
> >>  >  >  >  --
> >>  >  >  >  			Gleb.
> >>  >  >
> >>  >  >  Exit codes are also valid for r == -EINTR and -EAGAIN cases, eg
> >>  >  >  EXIT_INTR. Better print it out for all cases, and let the reader
> >>  >  >  decide whether exit_reason is valid.
> >>  >  Are they? I see that userspace does not look into run->exit_reason in
> >>  >  case of -EINTR and -EAGAIN.
> >>
> >>  Not userspace, but for a human reader. Otherwise the trace information
> >>  is incomplete and confusing.
> >But if we will always call trace_kvm_userspace_exit() here trace
> >information will show erroneous info since on any error but -EINTR
> >exit_reason contains stale info. What about changing r>= 0 to r>= 0 ||
> >r ==  -EINTR.
> 
> We should log both errno and exit_reason.  If we want to be clever,
> we can display strerror(errno) if it's nonzero, and exit_reason
> otherwise (easy to do in a trace-cmd plugin).
> 
For starters we should remove KVM_EXIT_INTR exit reason. Looking into
qemu-kvm history it was never used and there is at least one code path
that returns -EINTR and does not set KVM_EXIT_INTR, so exit_reason field
contains stale info on exit.

--
			Gleb.