RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doingguest device assignment

[Hao, Xudong]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Wednesday, May 09, 2012 10:34 AM
> To: Hao, Xudong
> Cc: Avi Kivity; Xudong Hao; mtosatti@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Zhang, Xiantao
> Subject: RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing guest
> device assignment
> 
> On Wed, 2012-05-09 at 01:18 +0000, Hao, Xudong wrote:
> > > -----Original Message-----
> > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > Sent: Tuesday, May 08, 2012 11:18 PM
> > > To: Hao, Xudong
> > > Cc: Avi Kivity; Xudong Hao; mtosatti@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > linux-kernel@xxxxxxxxxxxxxxx; Zhang, Xiantao
> > > Subject: RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing
> guest
> > > device assignment
> > >
> > > On Tue, 2012-05-08 at 09:16 +0000, Hao, Xudong wrote:
> > > > > -----Original Message-----
> > > > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > > > Sent: Tuesday, May 08, 2012 12:16 AM
> > > > > To: Hao, Xudong
> > > > > Cc: Avi Kivity; Xudong Hao; mtosatti@xxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx;
> > > > > linux-kernel@xxxxxxxxxxxxxxx; Zhang, Xiantao
> > > > > Subject: RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing
> > > guest
> > > > > device assignment
> > > > >
> > > > > On Mon, 2012-05-07 at 07:58 +0000, Hao, Xudong wrote:
> > > > > > > -----Original Message-----
> > > > > > > From: Avi Kivity [mailto:avi@xxxxxxxxxx]
> > > > > > > Sent: Sunday, May 06, 2012 11:34 PM
> > > > > > > To: Xudong Hao
> > > > > > > Cc: mtosatti@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > > > linux-kernel@xxxxxxxxxxxxxxx;
> > > > > > > Zhang, Xiantao; Hao, Xudong; Alex Williamson
> > > > > > > Subject: Re: [PATCH] kvm: Enable device LTR/OBFF capibility before
> doing
> > > > > guest
> > > > > > > device assignment
> > > > > > >
> > > > > > > On 05/06/2012 06:24 PM, Xudong Hao wrote:
> > > > > > > > Enable device LTR/OBFF capibility before do device assignment, so
> that
> > > > > guest
> > > > > > > can benefit from them.
> > > > > > >
> > > > > > > cc += Alex
> > > > > > >
> > > > > > > > @@ -166,6 +166,10 @@ int kvm_assign_device(struct kvm *kvm,
> > > > > > > >     if (pdev == NULL)
> > > > > > > >         return -ENODEV;
> > > > > > > >
> > > > > > > > +   /* Enable some device capibility before do device assignment,
> > > > > > > > +    * so that guest can benefit from them.
> > > > > > > > +    */
> > > > > > > > +   kvm_iommu_enable_dev_caps(pdev);
> > > > > > > >     r = iommu_attach_device(domain, &pdev->dev);
> > > > > > >
> > > > > > > Suppose we fail here.  Do we need to disable_dev_caps()?
> > > > > > >
> > > > >
> > > > > If kvm_assign_device() fails we'll try to restore the state we saved in
> > > > > kvm_vm_ioctl_assign_device(), so ltr/obff should be brought back to
> > > > > initial state.
> > > > >
> > > > Right, more clear.
> > > >
> > > > > > I don't think so. When a device will be assigned to guest, it's be
> > > > > > owned by a pci-stub driver, attach_device fail here do not affect
> > > > > > everything. If host want to use it, host device driver has its own
> > > > > > enable/disable dev_caps.
> > > > >
> > > > > Why is device assignment unique here?  If there's a default value that's
> > > > > known to be safe, why doesn't pci_enable_device set it for everyone?
> > > > > Host drivers can fine tune the value later if they want.
> > > > >
> >
> > If host did not have this device driver or host did not load the
> > driver, who will enable them? Guest? But in guest, it really need qemu
> > PCIe support.
> 
> The kvm driver does pci_enable_device(), just like any other PCI driver.
> If there's a safe default value, why isn't it set there?
> 

OK, I saw it. Seems it's reasonable to enable them in pci_enable_device(). I'll re-write patches there.

> > > > > > > >     if (r) {
> > > > > > > >         printk(KERN_ERR "assign device %x:%x:%x.%x failed",
> > > > > > > > @@ -228,6 +232,7 @@ int kvm_deassign_device(struct kvm
> *kvm,
> > > > > > > >         PCI_SLOT(assigned_dev->host_devfn),
> > > > > > > >         PCI_FUNC(assigned_dev->host_devfn));
> > > > > > > >
> > > > > > > > +   kvm_iommu_disable_dev_caps(pdev);
> > > > > > > >     return 0;
> > > > > > > >  }
> > > > > > > >
> > > > > > > > @@ -351,3 +356,30 @@ int kvm_iommu_unmap_guest(struct
> kvm
> > > *kvm)
> > > > > > > >     iommu_domain_free(domain);
> > > > > > > >     return 0;
> > > > > > > >  }
> > > > > > > > +
> > > > > > > > +static void kvm_iommu_enable_dev_caps(struct pci_dev *pdev)
> > > > > > > > +{
> > > > > > > > +   /* set default value */
> > > > > > > > +   unsigned long type = PCI_EXP_OBFF_SIGNAL_ALWAYS;
> > > > > > > > +   int snoop_lat_ns = 1024, nosnoop_lat_ns = 1024;
> > > > > > >
> > > > > > > Where does this magic number come from?
> > > > > > >
> > > > > > The number is the max value that register support, set it as default
> > > > > > here, we did not have any device here, and we do not know what's the
> > > > > > proper value, so it set a default value firstly.
> > > > >
> > > > > The register is composed of latency scale and latency value fields.
> > > > > 1024 is simply the largest value the latency value can hold (+1).  The
> > > > > scale field allows latencies up to 34,326,183,936ns to be specified, so
> > > > > please explain how 1024 is a universal default.
> > > > >
> > > >
> > > > Since each platform will have its own max supported latency, I think
> > > > the best way is setting the value to 0 because we have such a device
> > > > now.
> > >
> > > What's the benefit to that device vs the risk to other devices?
> >
> > Default value 0 does not affect any device, right?
> 
> I don't know, but if it doesn't affect any device, why bother?  On the
> risk side we have to question whether the device ltr/obff support works,
> whether the values we use are appropriate, whether the upstream
> interconnects support the same, and work, and whether the guest driver
> will behave appropriately with these enabled versus a gain of what?  So
> far it looks like we're turning it on simply because it's there.
> 
> > > Again,
> > > if there's a safe default value for both LTR and OBFF, why isn't PCI
> > > core setting it for everyone?  I'm inclined to wait for qemu express
> > > support and expose LTR/OBFF control to the guest if and only if we can
> > > enable it on the root complex and intermediate switches.  Thanks,
> > >
> >
> > Alex, do you means you're working on the qemu express support and
> > ltr/obff exposing? If so, when will this support finish?
> 
> Qemu express support is being worked on by the community, once
> available, it may be simple to expose these register if we determine
> it's safe for the guest to manipulate them.  I think there's a goal of
> incorporating express support by qemu 1.2, but I'm not driving it.
> Thanks,
> 
> Alex

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