Re: [PATCH RESEND v2 3/3] PCI: hv: New paravirtual PCI front-end for Hyper-V VMs

[Bjorn Helgaas]

On Tue, Feb 16, 2016 at 09:46:55PM +0000, Jake Oshins wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas [mailto:helgaas@xxxxxxxxxx]
> > Sent: Tuesday, February 16, 2016 8:46 AM
> > To: Jake Oshins <jakeo@xxxxxxxxxxxxx>
> > Cc: bhelgaas@xxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx;
> > gregkh@xxxxxxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>; linux-
> > kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> > apw@xxxxxxxxxxxxx; vkuznets@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; Haiyang
> > Zhang <haiyangz@xxxxxxxxxxxxx>; marc.zyngier@xxxxxxx; Hadden
> > Hoppert <haddenh@xxxxxxxxxxxxx>
> > Subject: Re: [PATCH RESEND v2 3/3] PCI: hv: New paravirtual PCI front-end
> > for Hyper-V VMs
> > 
> > Hi Jake,
> > 
> > Looks good to me overall; I marked a few nits below.
> > 
> > The only real question I have is about domain number allocation.  See
> > the note below.
> > 
> [snip]
> > > +
> > > +	/*
> > > +	 * The PCI bus "domain" is what is called "segment" in
> > > +	 * ACPI and other specs.  Pull it from the instance ID,
> > > +	 * to get something unique.  Bytes 8 and 9 are what is used
> > > +	 * in Windows guests, so do the same thing for consistency.
> > > +	 */
> > > +
> > > +	hbus->sysdata.domain = hdev->dev_instance.b[9] |
> > > +			       hdev->dev_instance.b[8] << 8;
> > 
> > How do we know this is unique?  We don't have any idea what the
> > platform will put in _SEG, so I think there's a potential conflict
> > here.  The Intel VMD driver (arch/x86/pci/vmd.c) has a similar
> > problem, and it looks for unused domain numbers starting at 0x10000
> > (see vmd_find_free_domain()).
> > 
> 
> Bjorn, thank you for your very thorough reviews.  I deeply appreciate it.  I checked the Hyper-V code and it currently does guarantee that these values are unique.  When I resend the series, I'll add a comment to that effect.  I'll also add a comment to Hyper-V that says that it has to stay that way.

I'm not familiar with how Hyper-V works, but I guess you're saying
that Hyper-V controls what the guest sees via _SEG as well as what it
sees via the instance ID.

Bjorn