linux-kernel - Re: [PATCH v5 3/4] kvm: Create kvm_clear

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Message-ID: <20120717210543.GA1868@redhat.com>
Date:	Wed, 18 Jul 2012 00:05:43 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	avi@...hat.com, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH v5 3/4] kvm: Create kvm_clear_irq()

On Tue, Jul 17, 2012 at 01:51:27PM -0600, Alex Williamson wrote:
> On Tue, 2012-07-17 at 21:55 +0300, Michael S. Tsirkin wrote:
> > On Tue, Jul 17, 2012 at 10:45:52AM -0600, Alex Williamson wrote:
> > > On Tue, 2012-07-17 at 19:21 +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Jul 17, 2012 at 10:17:03AM -0600, Alex Williamson wrote:
> > > > > > > > > > > >   And current code looks buggy if yes we need to fix it somehow.
> > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > Which to me seems to indicate this should be handled as a separate
> > > > > > > > > > > effort.
> > > > > > > > > > 
> > > > > > > > > > A separate patchset, sure. But likely a prerequisite: we still need to
> > > > > > > > > > look at all the code. Let's not copy bugs, need to fix them.
> > > > > > > > > 
> > > > > > > > > This looks tangential to me unless you can come up with an actual reason
> > > > > > > > > the above spinlock usage is incorrect or insufficient.
> > > > > > > > 
> > > > > > > > You copy the same pattern that seems racy. So you double the
> > > > > > > > amount of code that woul need to be fixed.
> > > > > > > 
> > > > > > > 
> > > > > > > _Seems_ racy, or _is_ racy?  Please identify the race.
> > > > > > 
> > > > > > Look at this:
> > > > > > 
> > > > > > static inline int kvm_irq_line_state(unsigned long *irq_state,
> > > > > >                                      int irq_source_id, int level)
> > > > > > {
> > > > > >         /* Logical OR for level trig interrupt */
> > > > > >         if (level)
> > > > > >                 set_bit(irq_source_id, irq_state);
> > > > > >         else
> > > > > >                 clear_bit(irq_source_id, irq_state);
> > > > > > 
> > > > > >         return !!(*irq_state);
> > > > > > }
> > > > > > 
> > > > > > 
> > > > > > Now:
> > > > > > If other CPU changes some other bit after the atomic change,
> > > > > > it looks like !!(*irq_state) might return a stale value.
> > > > > > 
> > > > > > CPU 0 clears bit 0. CPU 1 sets bit 1. CPU 1 sets level to 1.
> > > > > > If CPU 0 sees a stale value now it will return 0 here
> > > > > > and interrupt will get cleared.
> > > > > > 
> > > > > > 
> > > > > > Maybe this is not a problem. But in that case IMO it needs
> > > > > > a comment explaining why and why it's not a problem in
> > > > > > your code.
> > > > > 
> > > > > So you want to close the door on anything that uses kvm_set_irq until
> > > > > this gets fixed... that's insane.
> > > > 
> > > > What does kvm_set_irq use have to do with it?  You posted this patch:
> > > > 
> > > > +static int kvm_clear_pic_irq(struct kvm_kernel_irq_routing_entry *e,
> > > > +                            struct kvm *kvm, int irq_source_id)
> > > > +{
> > > > +#ifdef CONFIG_X86
> > > > +       struct kvm_pic *pic = pic_irqchip(kvm);
> > > > +       int level =
> > > > kvm_clear_irq_line_state(&pic->irq_states[e->irqchip.pin],
> > > > +                                            irq_source_id);
> > > > +       if (level)
> > > > +               kvm_pic_set_irq(pic, e->irqchip.pin,
> > > > +                               !!pic->irq_states[e->irqchip.pin]);
> > > > +       return level;
> > > > +#else
> > > > +       return -1;
> > > > +#endif
> > > > +}
> > > > +
> > > > 
> > > > it seems racy in the same way.
> > > 
> > > Now you're just misrepresenting how we got here, which was:
> > > 
> > > > > > > > > IMHO, we're going off into the weeds again with these last
> > > > > > > > > two patches.  It may be a valid optimization, but it really has no
> > > > > > > > > bearing on the meat of the series (and afaict, no significant
> > > > > > > > > performance difference either).
> > > > > > > > 
> > > > > > > > For me it's not a performance thing. IMO code is cleaner without this locking:
> > > > > > > > we add a lock but only use it in some cases, so the rules become really
> > > > > > > > complex.
> > > 
> > > So I'm happy to drop the last 2 patches, which were done at your request
> > > anyway, but you've failed to show how the locking in patches 1&2 is
> > > messy, inconsistent, or complex and now you're asking to block all
> > > progress.
> > 
> > I'm asking for bugs to get fixed and not duplicated. Adding more bugs is
> > not progress. Or maybe there is no bug. Let's see why and add a comment.
> > 
> > >  Those patches are just users of kvm_set_irq.
> > 
> > 
> > Well these add calls to kvm_set_irq which scans all vcpus under
> > spinlock. In the past Avi thought this is not a good idea too.
> > Maybe things changed.
> 
> We can drop the spinlock if we don't care about spurious EOIs, which is
> only a theoretical scalability problem anyway.

Not theoretical at all IMO. We see the problem with virtio with old
guests today.

> We're talking about
> level interrupts here, how scalable do we need to be?
> 

The reason we are moving them into kernel at all is for speed, no?

-- 
MST
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