linux-kernel - Re: [PATCH RFC] kvm: fast-path msi injection with irqfd

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Message-ID: <20101118143945.GR7948@redhat.com>
Date:	Thu, 18 Nov 2010 16:39:45 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Chris Lalancette <clalance@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: fast-path msi injection with irqfd

On Thu, Nov 18, 2010 at 03:48:43PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 18, 2010 at 03:35:01PM +0200, Gleb Natapov wrote:
> > On Thu, Nov 18, 2010 at 03:20:27PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 18, 2010 at 03:14:53PM +0200, Gleb Natapov wrote:
> > > > On Thu, Nov 18, 2010 at 03:03:37PM +0200, Michael S. Tsirkin wrote:
> > > > > > >+static inline void kvm_irq_routing_update(struct kvm *kvm,
> > > > > > >+					  struct kvm_irq_routing_table *irq_rt)
> > > > > > >+{
> > > > > > >+	rcu_assign_pointer(kvm->irq_routing, irq_rt);
> > > > > > >+}
> > > > > > >+
> > > > > > >  static inline int kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
> > > > > > >  {
> > > > > > >  	return -ENOSYS;
> > > > > > 
> > > > > > Apart from these minor issues, looks good.
> > > > > 
> > > > > 
> > > > > Something we should consider improving is the loop over all VCPUs that
> > > > > kvm_irq_delivery_to_apic invokes.  I think that (for non-broadcast
> > > > > interrupts) it should be possible to precompute an store the CPU
> > > > > in question as part of the routing entry.
> > > > > 
> > > > > Something for a separate patch ... comments?
> > > > > 
> > > > I do not think this info should be part of routing entry. Routing entry
> > > > is more about describing wires on the board.
> > > 
> > > Not for msi. kvm_kernel_irq_routing_entry seems to just keep an
> > > address/data pair in that case. So
> > > 
> > Yeah. Using routing_entry for MSI was miss design. We discussed that too :)
> > 
> > > 	union {
> > > 		struct {
> > > 			unsigned irqchip;
> > > 			unsigned pin;
> > > 		} irqchip;
> > > 		struct msi_msg msi;
> > > 	};
> > > 
> > > would become
> > > 
> > > 	union {
> > > 		struct {
> > > 			unsigned irqchip;
> > > 			unsigned pin;
> > > 		} irqchip;
> > > 		struct {
> > > 			struct msi_msg msi;
> > > 			struct kvm_vpcu *dest;
> > > 		} msi;
> > > 	};
> > > 
> > > or something like this.
> > Ah so you want to do it only for MSI? For MSI it makes sense. Remember
> > though that sometimes destination depend on message itself (specifically
> > on delivery mode).
> 
> Of course. We'll take message/data and precompute destination.
> Set to NULL for e.g. broadcast and recompute at injection time
> in that case.  BTW SELF doesn't work for MSI at the moment, not sure
> whether it's relevant or when is it used.
> 
Yes, only lowest prio is defined for MSI. Self or all but self has
not meaning for MSI.

--
			Gleb.
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