linux-kernel - Re: [Xen-devel] [PATCH] xen: reuse the same pirq allocated when driver load first time

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Date:	Thu, 23 May 2013 07:31:49 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	"Stefano Stabellini" <stefano.stabellini@...citrix.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"Chien Yen" <chien.yen@...cle.com>,
	"Feng Jin" <joe.jin@...cle.com>,
	"Yuval Shaia" <yuval.shaia@...cle.com>,
	"Zhenzhong Duan" <zhenzhong.duan@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] xen: reuse the same pirq allocated
 when driver load first time

>>> On 22.05.13 at 18:41, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> On Wed, May 22, 2013 at 04:25:10PM +0100, Jan Beulich wrote:
>> Okay, that clarifies it quite a bit. For one, I'll leave any of the
>> emuirq stuff to Stefano, who wrote this originally. And then, from
>> the beginning of this thread, I'm not convinced that freeing a pirq
>> is really the right thing here: unmap_pirq() is the counterpart of
>> map_pirq(), not get_free_pirq(). I would think that is a guest
>> allocates a pirq and then unmaps it without first mapping it, it's
>> the guest's fault that it now lost one pirq resource. It should not
>> have allocated one in the first place if it didn't mean to use it for
>> anything.
> 
> It does use it, but if you do run this in a loop:
>   rmmod e1000e;modprobe e1000e
> 
> it ends up doing thse three hypercalls: PHYSDEVOP_get_free_pirq,
> PHYSDEVOP_map_pirq, PHYSDEVOP_unmap_pirq and so on.

Yeah, my comment was more towards your simplified repro
module...

> The reason is that
> drivers/xen/events.c keeps track of the Linux IRQ <-> PIRQ just as long
> as needed - if the driver does a free_irq, well, then the mapping is
> de-allocated and lost.
> 
> One patch I posted (for Linux) keeps track of the PIRQ so that if
> free_irq is called and we remove the Linux IRQ <-> PIRQ association,
> we still have the PIRQ saved away and can re-use it.
> 
> In other words, the loop ends up doing:
>  PHYSDEVOP_map_pirq, PHYSDEVOP_unmap_pirq

And that's the right solution here imo. Remember that
get_free_pirq was introduced only for the pv-ops kernel, the
classic one never needed it (and hence couldn't leak pIRQ-s).

>> That setting to a negative value is not causing the slot to be
>> permanently lost, it merely defers its freeing. It was the only
>> way I could find back then to reasonably handle an unmap
>> being done before the matched unbind.
> 
> Ah, so pt_irq_destroy_bind (XEN_DOMCTL_unbind_pt_irq) is the counterpart
> to PHYSDEVOP_get_free_pirq in some form. Which looks to be on QEMU side
> only called when the PCI device is put in sleep or pulled out of the guest.
> 
> It probably shouldn't be called when the device is merely de-activated.

Right.

Jan

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