Alexey replied in the QEMU mailing list in his patch series. Here's the
link:
https://patchwork.kernel.org/cover/10767473/
Based on Alexey's explanation, I was able to create a quick PoC that changes
the calculation of the passthrough limit memory for the PPC64 machine in
qemu_domain.c, qemuDomainGetMemLockLimitBytes, to make the guest
boot up without IPC_LOCK privileges and with NVLink2 passthrough. Given
that this is based on how the NVLink2 support is implemented in the pseries
machine in QEMU, this code will need to verify the GPU model being
passed-through as well.
This deprecates the solution I was aiming for in this patch. There's no need
to grant IPC_LOCK for guests that uses VFIO because the guest will not
allocate RAM beyond the limit. This new approach is also confined in the
PPC64
code, thus the impact is minimal to x86 and other archs*.
Thanks,
DHB
* we can always generalize this code if other archs ends up doing DMA
resize with
passthrough devices the same way Power is doing.
On 2/6/19 3:05 PM, Daniel Henrique Barboza wrote:
On 2/6/19 2:34 PM, Alex Williamson wrote:
[cc +Alexey]
On Wed, 6 Feb 2019 13:44:53 -0200
Daniel Henrique Barboza <danielhb413@xxxxxxxxx> wrote:
QEMU virtual machines with PCI passthrough of certain devices,
such as the NVIDIA Tesla V100 GPU, requires allocation of an
amount of memory pages that can break KVM limits. When that
happens, the KVM module checks if the process is IPC_LOCK capable
and, in case it doesn't, it refuses to allocate the mem pages,
causing the guest to misbehave.
When Libvirt is compiled to use libcap-ng support, the resulting
QEMU guest Libvirt creates does not have CAP_IPC_LOCK, hurting
guests that are doing PCI passthrough and that requires extra
memory to work.
This patch addresses this issue by checking whether we're
running with libcap-ng support (WITH_CAPNG) and if the
guest is using PCI passthrough with the VFIO driver. If those
conditions are met, allow CAP_IPC_LOCK capability to the
QEMU process.
That effectively allows the QEMU process to lock all the memory it
wants and thereby generate a denial of service on the host, which is
the thing we're originally trying to prevent by accounting pinned pages
and enforcing the user's locked memory limits. The commit log doesn't
mention that this is only seen on POWER, but I infer that from the
previous thread. Does this happen for any assigned device on POWER or
is this something to do with the new NVLink support that Alexey has been
adding? Thanks,
You're right in both accounts. The behavior I am addressing here is
seen in a
Power 9 server using a QEMU build with NVLink2 passthrough support
from Alexey.
Not all assigned devices in Power wants to lock more memory than KVM
allows,
at least as far as I can tell.
About your argument of allowing QEMU to lock all memory, I agree that
is a problem. Aside from trusting that QEMU will not get greedy and lock
down all the host memory (which is what I'm doing here, in a sense), I
can only speculate alternatives. I'd rather wait for Alexey's input.
Thanks,
DHB
Alex
Signed-off-by: Daniel Henrique Barboza <danielhb413@xxxxxxxxx>
---
src/libvirt_private.syms | 1 +
src/qemu/qemu_process.c | 34 ++++++++++++++++++++++++++++++++++
src/util/virhostdev.c | 2 +-
src/util/virhostdev.h | 4 ++++
4 files changed, 40 insertions(+), 1 deletion(-)
diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 6b401aa5e0..b7384a67a8 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -1992,6 +1992,7 @@ virHostCPUStatsAssign;
# util/virhostdev.h
virHostdevFindUSBDevice;
+virHostdevGetPCIHostDeviceList;
virHostdevIsMdevDevice;
virHostdevIsSCSIDevice;
virHostdevManagerGetDefault;
diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 0583eb03f2..24aef5904f 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -4997,6 +4997,38 @@ qemuProcessSetupRawIO(virQEMUDriverPtr driver,
}
+static void
+qemuProcessSetupVFIOCaps(virDomainObjPtr vm ATTRIBUTE_UNUSED,
+ virCommandPtr cmd ATTRIBUTE_UNUSED)
+{
+#if WITH_CAPNG
+ virPCIDeviceListPtr pcidevs = NULL;
+ bool has_vfio = false;
+ size_t i;
+
+ pcidevs = virHostdevGetPCIHostDeviceList(vm->def->hostdevs,
+ vm->def->nhostdevs);
+ if (!pcidevs)
+ return;
+
+ for (i = 0; i < virPCIDeviceListCount(pcidevs); i++) {
+ virPCIDevicePtr pci = virPCIDeviceListGet(pcidevs, i);
+ if (virPCIDeviceGetStubDriver(pci) ==
VIR_PCI_STUB_DRIVER_VFIO) {
+ has_vfio = true;
+ break;
+ }
+ }
+
+ if (has_vfio) {
+ VIR_DEBUG("Adding CAP_IPC_LOCK to QEMU process");
+ virCommandAllowCap(cmd, CAP_IPC_LOCK);
+ }
+
+ virObjectUnref(pcidevs);
+#endif
+}
+
+
static int
qemuProcessSetupBalloon(virQEMUDriverPtr driver,
virDomainObjPtr vm,
@@ -6569,6 +6601,8 @@ qemuProcessLaunch(virConnectPtr conn,
if (qemuProcessSetupRawIO(driver, vm, cmd) < 0)
goto cleanup;
+ qemuProcessSetupVFIOCaps(vm, cmd);
+
virCommandSetPreExecHook(cmd, qemuProcessHook, &hookData);
virCommandSetMaxProcesses(cmd, cfg->maxProcesses);
virCommandSetMaxFiles(cmd, cfg->maxFiles);
diff --git a/src/util/virhostdev.c b/src/util/virhostdev.c
index 6be395cdda..ee389a1729 100644
--- a/src/util/virhostdev.c
+++ b/src/util/virhostdev.c
@@ -218,7 +218,7 @@ virHostdevManagerGetDefault(void)
return virObjectRef(manager);
}
-static virPCIDeviceListPtr
+virPCIDeviceListPtr
virHostdevGetPCIHostDeviceList(virDomainHostdevDefPtr *hostdevs,
int nhostdevs)
{
virPCIDeviceListPtr pcidevs;
diff --git a/src/util/virhostdev.h b/src/util/virhostdev.h
index 7263f320a2..9235581242 100644
--- a/src/util/virhostdev.h
+++ b/src/util/virhostdev.h
@@ -57,6 +57,10 @@ struct _virHostdevManager {
};
virHostdevManagerPtr virHostdevManagerGetDefault(void);
+virPCIDeviceListPtr
+virHostdevGetPCIHostDeviceList(virDomainHostdevDefPtr *hostdevs,
+ int nhostdevs)
+ ATTRIBUTE_NONNULL(1);
int
virHostdevPreparePCIDevices(virHostdevManagerPtr hostdev_mgr,
const char *drv_name,
--
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