[PATCH RESEND v2 3/3] PCI: hv: New paravirtual PCI front-end for Hyper-V VMs
Bjorn Helgaas
helgaas at kernel.org
Tue Feb 16 22:44:15 UTC 2016
On Tue, Feb 16, 2016 at 09:46:55PM +0000, Jake Oshins wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas [mailto:helgaas at kernel.org]
> > Sent: Tuesday, February 16, 2016 8:46 AM
> > To: Jake Oshins <jakeo at microsoft.com>
> > Cc: bhelgaas at google.com; linux-pci at vger.kernel.org;
> > gregkh at linuxfoundation.org; KY Srinivasan <kys at microsoft.com>; linux-
> > kernel at vger.kernel.org; devel at linuxdriverproject.org; olaf at aepfle.de;
> > apw at canonical.com; vkuznets at redhat.com; tglx at linutronix.de; Haiyang
> > Zhang <haiyangz at microsoft.com>; marc.zyngier at arm.com; Hadden
> > Hoppert <haddenh at microsoft.com>
> > 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
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