Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to call_rcu in vhost_set_memory() in vhost kernel module?

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 12, 2014 at 02:46:46PM +0200, Paolo Bonzini wrote:
> Il 12/05/2014 14:12, Michael S. Tsirkin ha scritto:
> >On Mon, May 12, 2014 at 01:46:19PM +0200, Paolo Bonzini wrote:
> >>Il 12/05/2014 13:07, Michael S. Tsirkin ha scritto:
> >>>On Mon, May 12, 2014 at 12:25:35PM +0200, Paolo Bonzini wrote:
> >>>>Il 12/05/2014 12:18, Michael S. Tsirkin ha scritto:
> >>>>>On Mon, May 12, 2014 at 12:14:25PM +0200, Paolo Bonzini wrote:
> >>>>>>Il 12/05/2014 12:08, Michael S. Tsirkin ha scritto:
> >>>>>>>On Mon, May 12, 2014 at 11:57:32AM +0200, Paolo Bonzini wrote:
> >>>>>>>>Perhaps we can check for cases where only the address is changing,
> >>>>>>>>and poke at an existing struct kvm_kernel_irq_routing_entry without
> >>>>>>>>doing any RCU synchronization?
> >>>>>>>
> >>>>>>>I suspect interrupts can get lost then: e.g. if address didn't match any
> >>>>>>>cpus, now it matches some. No?
> >>>>>>
> >>>>>>Can you explain the problem more verbosely? :)
> >>>>>>
> >>>>>>Multiple writers would still be protected by the mutex, so you
> >>>>>>cannot have an "in-place update" writer racing with a "copy the
> >>>>>>array" writer.
> >>>>>
> >>>>>I am not sure really.
> >>>>>I'm worried about reader vs writer.
> >>>>>If reader sees a stale msi value msi will be sent to a wrong
> >>>>>address.
> >>>>
> >>>>That shouldn't happen on any cache-coherent system, no?
> >>>>
> >>>>Or at least, it shouldn't become any worse than what can already
> >>>>happen with RCU.
> >>>
> >>>Meaning guest must do some synchronization anyway?
> >>
> >>Yes, I think so.  The simplest would be to mask the interrupt around
> >>MSI configuration changes.  Radim was looking at a similar bug.
> >
> >Was this with classic assignment or with vfio?
> 
> No, it was with QEMU emulated devices (AHCI).  Not this path, but it
> did involve MSI configuration changes while the interrupt was
> unmasked in the device.
> 
> >>He couldn't replicate on bare metal,
> >>but I don't see why it shouldn't
> >>be possible there too.
> >
> >I doubt linux does something tricky here, so
> >I'm not talking about something linux guest does,
> >I'm talking about an abstract guarantee of the
> >APIC.
> 
> APIC or PCI?  The chipset is involved here, not the APICs (the APICs
> just see interrupt transactions).
> 
> >If baremetal causes a synchronisation point
> >and we don't, at some point linux will use it and
> >this will bite us.
> 
> Yes, and I agree.
> 
> It's complicated; on one hand I'm not sure we can entirely fix this,
> on the other hand it may be that the guest cannot have too high
> expectations.
> 
> The invalid scenario is an interrupt happening before the MSI
> reconfiguration and using the new value.  It's invalid because MSI
> reconfiguration is a non-posted write and must push previously
> posted writes.  But we cannot do that at all!  QEMU has the big lock
> that implicitly orders transactions, but this does not apply to
> high-performance users of irqfd (vhost, VFIO, even QEMU's own
> virtio-blk dataplane).  Even if we removed all RCU uses for the
> routing table, we don't have a "bottleneck" that orders transactions
> and removes this invalid scenario.
> 
> At the same time, it's obviously okay if an interrupt happens after
> the MSI reconfiguration and uses the old value.  That's a posted
> write passing an earlier non-posted write, which is allowed.  This
> is the main reason why I believe that masking/unmasking is required
> on bare metal. In addition, if the masking/unmasking is done with a
> posted write (MMIO BAR), the guest needs to do a read for
> synchronization before it unmasks the interrupt.
> 
> In any case, whether writes synchronize with RCU or bypass it
> doesn't change the picture.  In either case, writes are ordered
> against each other but not against reads.  RCU does nothing except
> preventing dangling pointer accesses.
> 
> Paolo

This is the only part I don't get.
RCU will make sure no VCPUs are running, won't it?
So it's a kind of full barrier.

-- 
MST