linux-kernel - RE: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically

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Message-ID: <6E21E5352C11B742B20C142EB499E04801191C@TK5EX14MBXC124.redmond.corp.microsoft.com>
Date:	Mon, 21 Feb 2011 03:43:58 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"gregkh@...e.de" <gregkh@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"Hank Janssen" <hjanssen@...rosoft.com>
Subject: RE: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically



> -----Original Message-----
> From: Thomas Gleixner [mailto:tglx@...utronix.de]
> Sent: Sunday, February 20, 2011 11:16 AM
> To: KY Srinivasan
> Cc: gregkh@...e.de; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; virtualization@...ts.osdl.org; Haiyang Zhang; Hank
> Janssen
> Subject: RE: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically
> 
> On Sat, 19 Feb 2011, KY Srinivasan wrote:
> > > When grabbing some random irq from the PIC is not an issue, then
> > > what's the point of this probing, retry loop and the comments about
> > > racing ? What races here? That does not make sense at all.
> >
> > Like most virtualization platforms, Hyper-V also emulates the full PC
> > platform. So, it is possible that the driver of some other emulated
> > devices might register for the IRQ line we might have selected. That
> > is the race this code addresses. For performance reasons, we want
> > both storage and network traffic to go over the PV drivers.
> 
> So in case your driver gets the interrupt line first, which the other
> driver wants to acquire as well, then what? Do you want to do that
> probe magic in the other driver as well? What if this is a regular
> device driver which gets its irq number from ACPI/PCI or
> whatever. Then that driver simply wont work as it's interrupt line is
> busy.
> 
> > >
> > > I don't know why the previous reviewer wanted to have that
> > > dynamic. That just does not make sense to me.
> >
> > Prior to this patch, we had a hard coded interrupt line for use by
> > this driver. If that line was already in use, the load of this driver
> > would fail. This would be a fatal issue especially for distributions
> > that have embedded these PV drivers as part of their installation
> > media. This patch deals with such collisions in a more graceful way -
> > we would not bail until we have scanned all low interrupt lines.
> 
> So you trade breaking the PV stuff against breaking random other
> drivers? That doesn't sound like a brilliant idea.
> 
> There are various ways to solve that proper.
> 
>  - You can provide the interrupt number from ACPI/PCI or whatever your HV
>    provides as enumeration.
> 
>  - Use a fixed vector like XEN does for the event channel
> 
>  - Use dynamic allocation in the IOAPIC space like the kernel does for
>    MSI(-X)
> 
> Thanks,
> 
> 	tglx

I am not claiming that what I have done here is the best possible solution.
However, I will submit to you that it is better than what we had here
prior to this patch.  I will address this and a  whole lot of other issues 
in future patches. 

Regards,

K. Y

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