RE: [PATCH 4/4 RESEND] PCI: hv: Propagate coherence from VMbus device to PCI device

[Michael Kelley (LINUX)]

From: Robin Murphy <robin.murphy@xxxxxxx> Sent: Thursday, March 17, 2022 10:15 AM
> 
> On 2022-03-17 16:25, Michael Kelley via iommu wrote:
> > PCI pass-thru devices in a Hyper-V VM are represented as a VMBus
> > device and as a PCI device.  The coherence of the VMbus device is
> > set based on the VMbus node in ACPI, but the PCI device has no
> > ACPI node and defaults to not hardware coherent.  This results
> > in extra software coherence management overhead on ARM64 when
> > devices are hardware coherent.
> >
> > Fix this by propagating the coherence of the VMbus device to the
> > PCI device.  There's no effect on x86/x64 where devices are
> > always hardware coherent.
> >
> > Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > ---
> >   drivers/pci/controller/pci-hyperv.c | 17 +++++++++++++----
> >   1 file changed, 13 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> > index ae0bc2f..14276f5 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -49,6 +49,7 @@
> >   #include <linux/refcount.h>
> >   #include <linux/irqdomain.h>
> >   #include <linux/acpi.h>
> > +#include <linux/dma-map-ops.h>
> >   #include <asm/mshyperv.h>
> >
> >   /*
> > @@ -2142,9 +2143,9 @@ static void hv_pci_remove_slots(struct hv_pcibus_device
> *hbus)
> >   }
> >
> >   /*
> > - * Set NUMA node for the devices on the bus
> > + * Set NUMA node and DMA coherence for the devices on the bus
> >    */
> > -static void hv_pci_assign_numa_node(struct hv_pcibus_device *hbus)
> > +static void hv_pci_assign_properties(struct hv_pcibus_device *hbus)
> >   {
> >   	struct pci_dev *dev;
> >   	struct pci_bus *bus = hbus->bridge->bus;
> > @@ -2167,6 +2168,14 @@ static void hv_pci_assign_numa_node(struct
> hv_pcibus_device *hbus)
> >   				     numa_map_to_online_node(
> >   					     hv_dev->desc.virtual_numa_node));
> >
> > +		/*
> > +		 * On ARM64, propagate the DMA coherence from the VMbus device
> > +		 * to the corresponding PCI device. On x86/x64, these calls
> > +		 * have no effect because DMA is always hardware coherent.
> > +		 */
> > +		dev_set_dma_coherent(&dev->dev,
> > +			dev_is_dma_coherent(&hbus->hdev->device));
> 
> Eww... if you really have to do this, I'd prefer to see a proper
> hv_dma_configure() helper implemented and wired up to
> pci_dma_configure(). Although since it's a generic property I guess at
> worst pci_dma_configure could perhaps propagate coherency from the host
> bridge to its children by itself in the absence of any other firmware
> info. And it's built-in so could use arch_setup_dma_ops() like everyone
> else.
> 

I'm not seeing an existing mechanism to provide a "helper" or override
of pci_dma_configure().   Could you elaborate?  Or is this something
that needs to be created?

Michael

> Robin.
> 
> > +
> >   		put_pcichild(hv_dev);
> >   	}
> >   }
> > @@ -2191,7 +2200,7 @@ static int create_root_hv_pci_bus(struct hv_pcibus_device
> *hbus)
> >   		return error;
> >
> >   	pci_lock_rescan_remove();
> > -	hv_pci_assign_numa_node(hbus);
> > +	hv_pci_assign_properties(hbus);
> >   	pci_bus_assign_resources(bridge->bus);
> >   	hv_pci_assign_slots(hbus);
> >   	pci_bus_add_devices(bridge->bus);
> > @@ -2458,7 +2467,7 @@ static void pci_devices_present_work(struct work_struct
> *work)
> >   		 */
> >   		pci_lock_rescan_remove();
> >   		pci_scan_child_bus(hbus->bridge->bus);
> > -		hv_pci_assign_numa_node(hbus);
> > +		hv_pci_assign_properties(hbus);
> >   		hv_pci_assign_slots(hbus);
> >   		pci_unlock_rescan_remove();
> >   		break;