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

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Message-ID: <E959C4978C3B6342920538CF579893F0025534D5@SHSMSX104.ccr.corp.intel.com>
Date:	Tue, 26 May 2015 13:59:59 +0000
From:	"Wu, Feng" <feng.wu@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"joro@...tes.org" <joro@...tes.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"jiang.liu@...ux.intel.com" <jiang.liu@...ux.intel.com>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wu, Feng" <feng.wu@...el.com>
Subject: RE: [v7 4/8] iommu, x86: No need to migrating irq for VT-d
 Posted-Interrupts



> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Tuesday, May 26, 2015 6:00 PM
> To: Wu, Feng
> Cc: joro@...tes.org; dwmw2@...radead.org; jiang.liu@...ux.intel.com;
> iommu@...ts.linux-foundation.org; linux-kernel@...r.kernel.org
> Subject: RE: [v7 4/8] iommu, x86: No need to migrating irq for VT-d
> Posted-Interrupts
> 
> 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.

Oh, Yes, I didn't notice that they are both protected by that lock. In that case,
I can just add a filed like you mentioned below. Thanks for the comments!

Thanks,
Feng

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