Re: [Xen-devel] [PATCH v9 11/17] vt-d: Add API to update IRTE when VT-d PI is used

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Wu, Feng
> Sent: Tuesday, November 24, 2015 3:54 PM
> 
> 
> > -----Original Message-----
> > From: Tian, Kevin
> > Sent: Tuesday, November 24, 2015 3:45 PM
> > To: Wu, Feng <feng.wu@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> > Cc: Zhang, Yang Z <yang.z.zhang@xxxxxxxxx>; Keir Fraser <keir@xxxxxxx>; Jan
> > Beulich <jbeulich@xxxxxxxx>; Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Subject: RE: [PATCH v9 11/17] vt-d: Add API to update IRTE when VT-d PI is
> > used
> >
> > > +
> > > +    iommu = drhd->iommu;
> > > +    ir_ctrl = iommu_ir_ctrl(iommu);
> > > +    if ( !ir_ctrl )
> > > +        return -ENODEV;
> > > +
> > > +    spin_lock_irq(&ir_ctrl->iremap_lock);
> > > +
> > > +    GET_IREMAP_ENTRY(ir_ctrl->iremap_maddr, remap_index,
> > iremap_entries, p);
> > > +
> > > +    old_ire = *p;
> > > +
> > > +    /* Setup/Update interrupt remapping table entry. */
> > > +    setup_posted_irte(&new_ire, &old_ire, pi_desc, gvec);
> > > +    ret = cmpxchg16b(p, &old_ire, &new_ire);
> > > +
> > > +    /*
> > > +     * In the above, we use cmpxchg16 to atomically update the 128-bit
> > IRTE,
> > > +     * and the hardware cannot update the IRTE behind us, so the return
> > value
> >
> > hardware can DEFINITELY update IRTE behind us, right? e.g. after the IRTE
> > entry
> > is fully up, when interrupt is posted, etc. Here you might mean hardware
> > cannot
> > update the IRTE at this point?
> 
> Yes, you description above is more accurate. But why hardware needs to
> update IRTE when interrupt is posted? I think it needs to update the
> posted interrupt descriptor when posting an interrupt, not the IRTE,
> right?
> 

sorry mess IRTE and posted descriptor together. but using "behind"
is still not accurate to state your point here. :-)

Thanks
Kevin

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