[libvirt] [PATCH v1 1/1] qemu_process.c: add CAP_IPC_LOCK when using libcap-ng
Daniel Henrique Barboza
danielhb413 at gmail.com
Thu Feb 7 16:14:10 UTC 2019
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 at gmail.com> 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 at gmail.com>
>>> ---
>>> 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,
>
More information about the libvir-list
mailing list