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 > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
