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vfio/ccw/pci: Allow devices to opt-in for ballooning

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If a vfio assigned device makes use of a physical IOMMU, then memory
ballooning is necessarily inhibited due to the page pinning, lack of
page level granularity at the IOMMU, and sufficient notifiers to both
remove the page on balloon inflation and add it back on deflation.
However, not all devices are backed by a physical IOMMU.  In the case
of mediated devices, if a vendor driver is well synchronized with the
guest driver, such that only pages actively used by the guest driver
are pinned by the host mdev vendor driver, then there should be no
overlap between pages available for the balloon driver and pages
actively in use by the device.  Under these conditions, ballooning
should be safe.

vfio-ccw devices are always mediated devices and always operate under
the constraints above.  Therefore we can consider all vfio-ccw devices
as balloon compatible.

The situation is far from straightforward with vfio-pci.  These
devices can be physical devices with physical IOMMU backing or
mediated devices where it is unknown whether a physical IOMMU is in
use or whether the vendor driver is well synchronized to the working
set of the guest driver.  The safest approach is therefore to assume
all vfio-pci devices are incompatible with ballooning, but allow user
opt-in should they have further insight into mediated devices.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
This commit is contained in:
Alex Williamson 2018-08-17 09:27:16 -06:00
parent c65ee43315
commit 238e917285
5 changed files with 59 additions and 2 deletions
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26
hw/vfio/pci.c
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@ -2804,12 +2804,13 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
VFIOPCIDevice *vdev = DO_UPCAST(VFIOPCIDevice, pdev, pdev);
VFIODevice *vbasedev_iter;
VFIOGroup *group;
char *tmp, group_path[PATH_MAX], *group_name;
char *tmp, *subsys, group_path[PATH_MAX], *group_name;
Error *err = NULL;
ssize_t len;
struct stat st;
int groupid;
int i, ret;
bool is_mdev;
if (!vdev->vbasedev.sysfsdev) {
if (!(~vdev->host.domain || ~vdev->host.bus ||
@ -2869,6 +2870,27 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
}
}
/*
* Mediated devices *might* operate compatibly with memory ballooning, but
* we cannot know for certain, it depends on whether the mdev vendor driver
* stays in sync with the active working set of the guest driver. Prevent
* the x-balloon-allowed option unless this is minimally an mdev device.
*/
tmp = g_strdup_printf("%s/subsystem", vdev->vbasedev.sysfsdev);
subsys = realpath(tmp, NULL);
g_free(tmp);
is_mdev = (strcmp(subsys, "/sys/bus/mdev") == 0);
free(subsys);
trace_vfio_mdev(vdev->vbasedev.name, is_mdev);
if (vdev->vbasedev.balloon_allowed && !is_mdev) {
error_setg(errp, "x-balloon-allowed only potentially compatible "
"with mdev devices");
vfio_put_group(group);
goto error;
}
ret = vfio_get_device(group, vdev->vbasedev.name, &vdev->vbasedev, errp);
if (ret) {
vfio_put_group(group);
@ -3170,6 +3192,8 @@ static Property vfio_pci_dev_properties[] = {
DEFINE_PROP_BIT("x-igd-opregion", VFIOPCIDevice, features,
VFIO_FEATURE_ENABLE_IGD_OPREGION_BIT, false),
DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),
DEFINE_PROP_BOOL("x-balloon-allowed", VFIOPCIDevice,
vbasedev.balloon_allowed, false),
DEFINE_PROP_BOOL("x-no-kvm-intx", VFIOPCIDevice, no_kvm_intx, false),
DEFINE_PROP_BOOL("x-no-kvm-msi", VFIOPCIDevice, no_kvm_msi, false),
DEFINE_PROP_BOOL("x-no-kvm-msix", VFIOPCIDevice, no_kvm_msix, false),