Re: 0xB16B00B5? Really? (was Re: Move hyperv out of the drivers/staging/directory)

[Anthony Liguori]

On 07/19/2012 05:30 PM, KY Srinivasan wrote:
> > -----Original Message-----
> > From: Greg KH (gregkh@xxxxxxxxxxxxxxxxxxx)
> > [mailto:gregkh@xxxxxxxxxxxxxxxxxxx]
> > Sent: Thursday, July 19, 2012 6:02 PM
> > To: KY Srinivasan
> > Cc: Paolo Bonzini; devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > virtualization@xxxxxxxxxxxxxx
> > Subject: Re: 0xB16B00B5? Really? (was Re: Move hyperv out of the
> > drivers/staging/ directory)
> >
> > On Thu, Jul 19, 2012 at 09:22:53PM +0000, KY Srinivasan wrote:
> > > > -----Original Message-----
> > > > From: Greg KH (gregkh@xxxxxxxxxxxxxxxxxxx)
> > > > [mailto:gregkh@xxxxxxxxxxxxxxxxxxx]
> > > > Sent: Thursday, July 19, 2012 5:07 PM
> > > > To: KY Srinivasan
> > > > Cc: Paolo Bonzini; devel@xxxxxxxxxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx;
> > > > virtualization@xxxxxxxxxxxxxx
> > > > Subject: Re: 0xB16B00B5? Really? (was Re: Move hyperv out of the
> > > > drivers/staging/ directory)
> > > >
> > > > On Thu, Jul 19, 2012 at 02:11:47AM +0000, KY Srinivasan wrote:
> > > > > > -----Original Message-----
> > > > > > From: Paolo Bonzini [mailto:paolo.bonzini@xxxxxxxxx] On Behalf Of Paolo
> > > > > > Bonzini
> > > > > > Sent: Friday, July 13, 2012 6:23 AM
> > > > > > To: KY Srinivasan
> > > > > > Cc: Greg KH; devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > > > > virtualization@xxxxxxxxxxxxxx
> > > > > > Subject: 0xB16B00B5? Really? (was Re: Move hyperv out of the
> > > > drivers/staging/
> > > > > > directory)
> > > > > >
> > > > > > Il 04/10/2011 21:34, Greg KH ha scritto:
> > > > > > > diff --git a/drivers/staging/hv/hyperv_vmbus.h
> > > > b/drivers/hv/hyperv_vmbus.h
> > > > > > > similarity index 99%
> > > > > > > rename from drivers/staging/hv/hyperv_vmbus.h
> > > > > > > rename to drivers/hv/hyperv_vmbus.h
> > > > > > > index 3d2d836..8261cb6 100644
> > > > > > > --- a/drivers/staging/hv/hyperv_vmbus.h
> > > > > > > +++ b/drivers/hv/hyperv_vmbus.h
> > > > > > > @@ -28,8 +28,7 @@
> > > > > > >   #include<linux/list.h>
> > > > > > >   #include<asm/sync_bitops.h>
> > > > > > >   #include<linux/atomic.h>
> > > > > > > -
> > > > > > > -#include "hyperv.h"
> > > > > > > +#include<linux/hyperv.h>
> > > > > > >
> > > > > > >   /*
> > > > > > >    * The below CPUID leaves are present if
> > > > > > VersionAndFeatures.HypervisorPresent
> > > > > >
> > > > > > git's rename detection snips away this gem:
> > > > > >
> > > > > > +#define HV_LINUX_GUEST_ID_LO		0x00000000
> > > > > > +#define HV_LINUX_GUEST_ID_HI		0xB16B00B5
> > > > > > +#define HV_LINUX_GUEST_ID		(((u64)HV_LINUX_GUEST_ID_HI
> > > > > > <<  32) | \
> > > > > > +					   HV_LINUX_GUEST_ID_LO)
> > > > > >
> > > > > > Somone was trying to be funny, I guess.
> > > > > >
> > > > > > KY, I suppose you have access to Hyper-V code or can ask someone who
> > > > does.
> > > > > > Is this signature actually used in the Hyper-V host code?
> > > > >
> > > > > Paolo,
> > > > >
> > > > > As I noted earlier, this is just a guest ID that needs to be registered with the
> > > > > hypervisor.  Thanks  for reporting this issue and on behalf of Microsoft, I
> > would
> > > > > like to  apologize for this offensive string. I have submitted a patch to fix this
> > > > issue.
> > > >
> > > > You only changed it to be in decimal, you did not change the id at all.
> > > > Is there some reason why you can not change it?  You said there was a
> > > > reserved range of ids that could be used, perhaps just pick another one?
> > > > What is the valid range that can be used here?
> > >
> > > Greg,
> > >
> > > As you know, this ID has been in use for a long time now. While the hypervisor
> > > does not interpret the guest ID that is registered, I am not sure what
> > dependencies
> > > there might be on this value.
> >
> > Could you please go find out the answer to this?
>
> That is easier said than done. I have sent emails out asking this very question and I have
> not received a definitive answer yet. Not knowing if and when I can get a definitive
> answer here, I chose the least risky approach in my patch.
> > If, as you originally stated, there is a range of values we can use,
> > then we should probably use another one, right?
>
> On the Windows side this ID namespace is managed well. However on the Linux
> side, we have really had this current ID in use for almost five years now. I am not
> aware of any pool of IDs available for Linux usage except that Linux IDs be distinct from
> the guest IDs in use by MSFT operating systems. If I were to change the guest ID, I would
> probably want to comply with the MSFT guidance on constructing these IDs (although not
> all fields may be relevant for Linux).

Presumably, Hyper-V can deal with unexpected values here, no?  Otherwise, it 
wouldn't be future proof against new types of guests.

So worst case scenario, Hyper-V disables optimizations on Linux guests that 
report then new ID until they patch Hyper-V to know about the new ID.

That seems like a reasonable trade off to me.  I'm sure there's sufficient 
incentive to patch Hyper-V for this at Microsoft...

Regards,

Anthony Liguori

> Regards,
>
> K. Y

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