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

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Date:	Mon, 21 Feb 2011 15:51:56 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	KY Srinivasan <kys@...rosoft.com>
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

On Mon, 21 Feb 2011, KY Srinivasan wrote:
> > > > 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.
> > 
> > No, it's _NOT_ better in any way. You trade breaking your PV thing
> > against breaking random other drivers. Care to explain why you think
> > that's better ?
> 
> The root device for the VM is bound to the PV driver on some distributions.
> So, if we cannot load the PV drivers, we do not have a system that boots.
> In general, the system performance without these PV drivers is so poor that
> for all practical purposes, having the PV drivers is almost a requirement
> for having a useable system. While the platform supports configuration of the VM with
> some emulated devices, it is not a recommended configuration (because of
> performance reasons) for Linux virtual machines on the Hyper-V platform.

It does not matter whether it's recommended or not. If it results in a
non usable emulated device it's broken. Just imagine you grab the
serial interrupt, which results in not having a debug console. Not at
all acceptable.
 
> I have spent significantly more time debating this patch than
> developing this patch that I still think improves the current
> driver. I will leave it to Greg and other powers that be to decide
> if this patch will be accepted. Let me know what your verdict is.

As I said, any other clean solution (and I pointed out 3) over this
hack.

Thanks,

	tglx
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