Re: [RFC v2 PATCH 2/7] KVM: Track the pid of the VM process

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Sep 06, 2016 at 04:22:17PM +0100, Punit Agrawal wrote:
> Christoffer Dall <christoffer.dall@linaro.org> writes:
> 
> > On Tue, Sep 06, 2016 at 12:07:59PM +0100, Punit Agrawal wrote:
> >> Christoffer Dall <christoffer.dall@linaro.org> writes:
> >> 
> >> > On Tue, Sep 06, 2016 at 10:51:27AM +0100, Punit Agrawal wrote:
> >> >> Hi Christoffer,
> >> >> 
> >> >> Christoffer Dall <christoffer.dall@linaro.org> writes:
> >> >> 
> >> >> > On Mon, Sep 05, 2016 at 05:31:32PM +0100, Punit Agrawal wrote:
> >> >> >> Userspace tools such as perf can be used to profile individual
> >> >> >> processes.
> >> >> >> 
> >> >> >> Track the PID of the virtual machine process to match profiling requests
> >> >> >> targeted at it. This can be used to take appropriate action to enable
> >> >> >> the requested profiling operations for the VMs of interest.
> >> >> >> 
> >> >> >> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> >> >> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> >> >> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
> >> >> >> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> >> >> >> Cc: Marc Zyngier <marc.zyngier@arm.com>
> >> >> >> ---
> >> >> >>  include/linux/kvm_host.h | 1 +
> >> >> >>  virt/kvm/kvm_main.c      | 2 ++
> >> >> >>  2 files changed, 3 insertions(+)
> >> >> >> 
> >> >> >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> >> >> >> index 9c28b4d..7c42c94 100644
> >> >> >> --- a/include/linux/kvm_host.h
> >> >> >> +++ b/include/linux/kvm_host.h
> >> >> >> @@ -374,6 +374,7 @@ struct kvm_memslots {
> >> >> >>  struct kvm {
> >> >> >>  	spinlock_t mmu_lock;
> >> >> >>  	struct mutex slots_lock;
> >> >> >> +	struct pid *pid;
> >> >> >>  	struct mm_struct *mm; /* userspace tied to this vm */
> >> >> >>  	struct kvm_memslots *memslots[KVM_ADDRESS_SPACE_NUM];
> >> >> >>  	struct srcu_struct srcu;
> >> >> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >> >> >> index 1950782..ab2535a 100644
> >> >> >> --- a/virt/kvm/kvm_main.c
> >> >> >> +++ b/virt/kvm/kvm_main.c
> >> >> >> @@ -613,6 +613,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
> >> >> >>  	spin_lock_init(&kvm->mmu_lock);
> >> >> >>  	atomic_inc(&current->mm->mm_count);
> >> >> >>  	kvm->mm = current->mm;
> >> >> >> +	kvm->pid = get_task_pid(current, PIDTYPE_PID);
> >> >> >
> >> >> > How dooes this deal with threading?  Is the idea that the user by
> >> >> > specifying the main thread's pid will enable trapping for all vcpu
> >> >> > threads belonging to that VM?
> >> >> 
> >> >> Yes that's correct - specifying the main thread PID will enable trapping
> >> >> for the VM (all vcpus).
> >> >> 
> >> >> I am happy to move to a more suitable identifier if available.
> >> >> 
> >> >
> >> > What is the 'main thread' ?
> >> >
> >> > Does something mandate that the VM is created by the thread group
> >> > leader?  If not, is it not a bit strange from a user perspective, that
> >> > you have to find the specific subthread pid that created the vm to
> >> > enable this tracing for all vcpu threads and that the tgid doesn't work
> >> > in this case?
> >> 
> >> Let me correct my terminology usage - the value recorded above (and used
> >> to identify the VM) should be the tgid. It is confusing because 'ps'
> >> reports it as pid.
> >> 
> >> I picked the value as existing KVM code already uses the PID of the
> >> creating task (see kvm_create_vm_debugfs) to export VM statistics in
> >> debugfs.
> >> 
> >> If I've got this wrong, then kvm_create_vm_debugfs also likely needs an
> >> update.
> >> 
> >> What do you think?
> >> 
> > When you do get_task_pid(current, PIDTYPE_PID) it actually gets the
> > kernel view of a PID which is the thead-id from userspace's point of
> > view, right?
> 
> That makes sense. It seems to works here because the pid of the first
> task is also the tgid of the group. And I reckon it's the same
> assumption being made with debugfs code (more below).

That is probably the implementation of all QEMU versions and kvmtool
versions out there.

> 
> I've changed the first argument of the call to get_task_pid to
> current->group_leader.
> 
> >
> > I don't see why this has to be the same as the debugfs code, as there it
> > makes potentially more sense to thread-specific, but for your case, are
> > you not targeting the behavior that a user can do "ps aux | grep qemu"
> > or whatever, and then set tracing for the reported PID (which is
> > actually a tgid)?
> 
> The debugfs stats are not thread (vcpu) specific but for the VM.
> 
> Both values, debugfs and here, are being used to represent the VM to the
> user. A mismatch in these identifiers will be very confusing.
> 
> If you agree, I can separately send a patch to address this for VM
> debugfs directory as well.
> 

I don't know how the debugfs stuff is used or was intended, so I really
can't speak for that.  It seems less weird to me with debugfs, because I
imagine it can be used by simply looking at what exists in the debugfs
directory and mapping that to a VM.

In your case, there's a clear expectation from the user that using the
tgid should cover this VM, and it will be weird if that's not the case.

-Christoffer