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

[Alex Williamson]

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 (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?  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

> > > > > +
> > > > > +   /* LTR(Latency tolerance reporting) allows devices to send
> > > > > +    * messages to the root complex indicating their latency
> > > > > +    * tolerance for snooped & unsnooped memory transactions.
> > > > > +    */
> > > > > +   pci_enable_ltr(pdev);
> > > > > +   pci_set_ltr(pdev, snoop_lat_ns, nosnoop_lat_ns);
> > > > > +
> > > > > +   /* OBFF (optimized buffer flush/fill), where supported,
> > > > > +    * can help improve energy efficiency by giving devices
> > > > > +    * information about when interrupts and other activity
> > > > > +    * will have a reduced power impact.
> > > > > +    */
> > > > > +   pci_enable_obff(pdev, type);
> > > > > +}
> > > > > +
> > > > > +static void kvm_iommu_disable_dev_caps(struct pci_dev *pdev)
> > > > > +{
> > > > > +   pci_disble_obff(pdev);
> > > > > +   pci_disble_ltr(pdev);
> > > > > +}
> > > >
> > > > Do we need to communicate something about these capabilities to the
> > guest?
> > > >
> > >
> > > I guess you means that here host don't know if guest want to enable them,
> > right?
> > > The ltr/obff new feature are supposed to enabled by guest if platform and
> > device supported.
> > 
> > It looks like ltr is a two part mechanism, the capability and enable
> > lives in the pci express capability, but the tuning registers live in
> > extended capability space.  The guest doesn't yet have access to the
> > latter since we don't have an express chipset.  The capability and
> > enable are read-only to the guest currently, same for obff.  Thanks,
> > 
> > Alex
> 



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