RE: [v7 4/8] iommu, x86: No need to migrating irq for VT-d Posted-Interrupts

[Thomas Gleixner <tglx@linutronix.de>]

On Tue, 26 May 2015, Wu, Feng wrote:
> > On Mon, 25 May 2015, Feng Wu wrote:
> > > +
> > > +	/* We don't need to modify irte if the interrupt is for posting. */
> > > +	if (irte->pst != 1)
> > > +		modify_irte(&ir_data->irq_2_iommu, irte);
> > 
> > I don't think this is correct. ir_data->irte_entry contains the non
> > posted version, which has pst == 0.
> > 
> > You need some other way to store whether you are in posted mode or
> > not.
> 
> Yes, seems this is incorrect. Thank you for pointing this out. After more
> thinking about this, I think I can do it this way:
> #1. Check the 'pst' field in hardware
> #2. If 'pst' is 1, we don't update the IRTE in hardware.
> 
> However, the question is the check and update operations should be protected
> by the same spinlock ' irq_2_ir_lock ', otherwise, race condition may happen.

Why? 

set_affinity() and vcpu_set_affinity() are serialized via
irq_desc->lock. And vcpu_set_affinity() is the only way to switch from
and to posted mode.

So all you need is a field in intel_irq_data which captures whether
posted is enabled or not.

Thanks,

	tglx