We can't afford the overhead of switching out and back into mmap mode
around each interrupt, but we can do it lazily via a timer. On INTx
interrupt, disable the mmap'd memory regions and set a timer. On
every interrupt, push the timer out. If the timer expires and the
interrupt is no longer pending, switch back to mmap mode.
This has the benefit that things like graphics cards, which rarely or
never, fire an interrupt don't need manual user intervention to add
the x-intx=off parameter. They'll just remain in mmap mode until they
trigger an interrupt, and if they don't continue to regularly fire
interrupts, they'll switch back.
The default timeout is tuned for network cards so that a ping is just
enough to keep them in non-mmap mode, where they have much better
latency. It is tunable with an experimental option,
x-intx-mmap-timeout-ms. A value of 0 keeps the device in non-mmap
mode after the first interrupt.
It's possible we could look at the class code of devices and come up
with reasonable per-class defaults based on expected interrupt
frequency and latency. None of this is used for MSI interrupts and
also won't be used if we can bypass through KVM.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
We cannot cast directly from pointer to uint64.
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Alex Barcelo <abarcelo@ac.upc.edu>
Reported-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
This adds the core of the QEMU VFIO-based PCI device assignment driver.
To make use of this driver, enable CONFIG_VFIO, CONFIG_VFIO_IOMMU_TYPE1,
and CONFIG_VFIO_PCI in your host Linux kernel config. Load the vfio-pci
module. To assign device 0000:05:00.0 to a guest, do the following:
for dev in $(ls /sys/bus/pci/devices/0000:05:00.0/iommu_group/devices); do
vendor=$(cat /sys/bus/pci/devices/$dev/vendor)
device=$(cat /sys/bus/pci/devices/$dev/device)
if [ -e /sys/bus/pci/devices/$dev/driver ]; then
echo $dev > /sys/bus/pci/devices/$dev/driver/unbind
fi
echo $vendor $device > /sys/bus/pci/drivers/vfio-pci/new_id
done
See Documentation/vfio.txt in the Linux kernel tree for further
description of IOMMU groups and VFIO.
Then launch qemu including the option:
-device vfio-pci,host=0000:05:00.0
Legacy PCI interrupts (INTx) currently makes use of a kludge where we
trap BAR accesses and assume the access is in response to an interrupt,
therefore de-asserting and unmasking the interrupt. It's not quite as
targetted as using the EOI for this, but it's self contained and seems
to work across all architectures. The side-effect is a significant
performance slow-down for device in INTx mode. Some devices, like
graphics cards, don't really use their interrupt, so this can be turned
off with the x-intx=off option, which disables INTx alltogether. This
should be considered an experimental option until we refine this code.
Both MSI and MSI-X are supported and avoid these issues.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
