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

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Message-ID: <6E21E5352C11B742B20C142EB499E048011693@TK5EX14MBXC124.redmond.corp.microsoft.com>
Date:	Sat, 19 Feb 2011 00:56:16 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Greg KH <gregkh@...e.de>
CC:	Greg KH <greg@...ah.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>
Subject: RE: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically



> -----Original Message-----
> From: Greg KH [mailto:gregkh@...e.de]
> Sent: Friday, February 18, 2011 5:29 PM
> To: KY Srinivasan
> Cc: Greg KH; linux-kernel@...r.kernel.org; devel@...uxdriverproject.org;
> virtualization@...ts.osdl.org
> Subject: Re: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically
> 
> On Fri, Feb 18, 2011 at 10:16:05PM +0000, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Greg KH [mailto:gregkh@...e.de]
> > > Sent: Friday, February 18, 2011 5:07 PM
> > > To: KY Srinivasan
> > > Cc: Greg KH; linux-kernel@...r.kernel.org; devel@...uxdriverproject.org;
> > > virtualization@...ts.osdl.org
> > > Subject: Re: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically
> > >
> > > On Fri, Feb 18, 2011 at 10:00:04PM +0000, KY Srinivasan wrote:
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Greg KH [mailto:greg@...ah.com]
> > > > > Sent: Friday, February 18, 2011 4:14 PM
> > > > > To: KY Srinivasan
> > > > > Cc: gregkh@...e.de; linux-kernel@...r.kernel.org;
> > > > > devel@...uxdriverproject.org; virtualization@...ts.osdl.org
> > > > > Subject: Re: [PATCH]: Staging: hv: Allocate the vmbus irq dynamically
> > > > >
> > > > > On Tue, Feb 15, 2011 at 11:55:35AM -0800, K. Y. Srinivasan wrote:
> > > > > >
> > > > > > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > > > > > Signed-off-by: Haiyang Zhang <haiyangz@...rosoft.com>
> > > > > > Signed-off-by: Hank Janssen <hjanssen@...rosoft.com>
> > > > >
> > > > > You didn't run this through checkpatch.pl.
> > > > >
> > > > > Please do so and fix the warning it gives you.
> > > > Greg, I did run the checkpatch script against this patch and the only
> > > > complaint I got was with regards to the IRQF_SAMPLE_RANDOM flag that I
> > > > pass. As a virtual machine, this is the only external event that the
> > > > VM is going to see and so I chose to keep this flag.  Is there
> > > > something that would replace this flag; looking at the Xen drivers
> > > > they do pass this flag.
> > >
> > > But that flag is going away, right?  And this really can't be a valid
> > > source of entropy as the HV channel is pretty predictable.
> >
> > Is it going away? What would replace this. Is all interrupt sources considered
> > predictable?
> 
> Did you read the file that the checkpatch script told you to about this
> entry?

It is only after reading the document, I decided to keep that flag. Please
note this is not a question of some interrupt sources not being 
a good source of entropy; for this VM this is the only source of interrupts.
The document on this flag talked about how people were incorrectly
marking their interrupt as an entropy source; in this case there is not much of a
choice. 

> 
> > This is the only unpredictable thing happening in the VM and that is the reason
> > I chose to keep the flag.
> 
> If you remove it, do we loose all entropy for the VM?
> 
> > > If you are only using this because Xen does/did it, that's not a valid
> > > excuse :)
> > Surely, you are joking.
> 
> Not at all.

To set the record straight here, this flag is in the existing code.
After I ran checkpatch, I toyed with the idea of getting rid of this. Then I 
decided to keep it for all the reasons I mentioned earlier. 

> 
> > In any event I am sending you a new patch with that flag removed.
> 
> Have you tested to see if you now loose all entropy, and it causes
> problems or not?

I am glad you asked me to test it. When I remove this flag, the entropy goes down 
significantly and this is not surprising. Looking at 
/proc/sys/kernel/entropy/entropy_avail, with the original patch after a couple
of compiles the number would be in thousands. With that flag removed,
I have the VM up for about an hour, even after a couple of compiles,
the entropy number is yet to crack 200.

Let me know how you want to proceed here.

Regards,

K. Y


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