linux-kernel - Re: [Xen-devel] [PATCH 2/3 v3] Refactor MSI restore call-chain to drop unnecessary argument

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Date:	Thu, 29 Aug 2013 08:23:08 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<zhenzhong.duan@...cle.com>
Cc:	"Bjorn Helgaas" <bhelgaas@...gle.com>,
	"xen-devel" <xen-devel@...ts.xenproject.org>,
	"Feng Jin" <joe.jin@...cle.com>,
	"Sucheta Chakraborty" <sucheta.chakraborty@...gic.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/3 v3] Refactor MSI restore call-chain
 to drop unnecessary argument

>>> On 29.08.13 at 04:52, Zhenzhong Duan <zhenzhong.duan@...cle.com> wrote:
> But in initial domain (aka priviliged guest), it's different.
> Driver init call graph under initial domain:
> driver_init->
>     msix_capability_init->
>         msix_program_entries->
>             msix_mask_irq->
>                 entry->masked = 1
>     request_irq->
>         __setup_irq->
>             irq_startup->
>                 __startup_pirq->
>                     EVTCHNOP_bind_pirq hypercall    (trap into Xen)
> [Xen:]
> pirq_guest_bind->
>     startup_msi_irq->
>         unmask_msi_irq->
>             msi_set_mask_bit->
>                 entry->msi_attrib.masked = 0
> 
> So entry->msi_attrib.masked in xen side always has newest value. entry->masked
> in initial domain is untouched and is 1 after msix_capability_init.

And as said several times before - Linux shouldn't be touching
the MSI-X table at all during initial setup or resume (it should in
particular not rely on such accesses to not fault, as being a
privilege violation); all it needs to do is update its software state.

Hence fiddling with default_restore_msi_irqs() seems the wrong
approach towards solving the problem.

Jan

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