Re: [PATCH] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending

[Seth Forshee <sforshee@digitalocean.com>]

On Tue, May 03, 2022 at 02:17:53PM +0000, Sean Christopherson wrote:
> On Tue, May 03, 2022, Seth Forshee wrote:
> > A livepatch migration for a task can only happen when the task is
> > sleeping or it exits to userspace. This may happen infrequently for a
> > heavily loaded vCPU task, leading to livepatch transition failures.
> > 
> > Fake signals will be sent to tasks which fail to migrate via stack
> > checking. This will cause running vCPU tasks to exit guest mode, but
> > since no signal is pending they return to guest execution without
> > exiting to userspace. Fix this by treating a pending livepatch migration
> > like a pending signal, exiting to userspace with EINTR. This allows the
> > migration to complete, and userspace should re-excecute KVM_RUN to
> > resume guest execution.
> > 
> > In my testing, systems where livepatching would timeout after 60 seconds
> > were able to load livepatches within a couple of seconds with this
> > change.
> > 
> > Signed-off-by: Seth Forshee <sforshee@digitalocean.com>
> > ---
> >  kernel/entry/kvm.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/entry/kvm.c b/kernel/entry/kvm.c
> > index 9d09f489b60e..efe4b791c253 100644
> > --- a/kernel/entry/kvm.c
> > +++ b/kernel/entry/kvm.c
> > @@ -14,7 +14,12 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work)
> >  				task_work_run();
> >  		}
> >  
> > -		if (ti_work & _TIF_SIGPENDING) {
> > +		/*
> > +		 * When a livepatch migration is pending, force an exit to
> 
> Can the changelog and comment use terminology other than migration?  Maybe "transition"?
> That seems to be prevelant through the livepatch code and docs.  There are already
> too many meanings for "migration" in KVM, e.g. live migration, page migration, task/vCPU
> migration, etc...

"Transition" is used a lot, but afaict it refers to the overall state of
the livepatch. "Migrate" is used a lot less, but it always seems to
refer to patching a single task, which is why I used that term. But I
can see the opportunity for confusion, so I'll reword it.

> 
> > +		 * userspace as though a signal is pending to allow the
> > +		 * migration to complete.
> > +		 */
> > +		if (ti_work & (_TIF_SIGPENDING | _TIF_PATCH_PENDING)) {
> 
> _TIF_PATCH_PENDING needs to be in XFER_TO_GUEST_MODE_WORK too, otherwise there's
> no guarantee KVM will see the flag and invoke xfer_to_guest_mode_handle_work().

Yes, you are right. I was relying on the livepatch code setting
_TIF_NOTIFY_SIGNAL for vCPU tasks which were running, but it would be
better to have _TIF_PATCH_PENDING in XFER_TO_GUEST_MODE_WORK too.

Thanks,
Seth

> 
> >  			kvm_handle_signal_exit(vcpu);
> >  			return -EINTR;
> >  		}
> > -- 
> > 2.32.0
> >