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Re: [PATCH] target/ppc: handle vcpu hotplug failure gracefully
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From: |
Harsh Prateek Bora |
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Subject: |
Re: [PATCH] target/ppc: handle vcpu hotplug failure gracefully |
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Date: |
Mon, 20 May 2024 15:25:51 +0530 |
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User-agent: |
Mozilla Thunderbird |
On 5/17/24 09:30, Nicholas Piggin wrote:
On Thu May 16, 2024 at 2:31 PM AEST, Harsh Prateek Bora wrote:
Hi Nick,
On 5/14/24 08:39, Nicholas Piggin wrote:
On Tue Apr 23, 2024 at 4:30 PM AEST, Harsh Prateek Bora wrote:
+ qemu-devel
On 4/23/24 11:40, Harsh Prateek Bora wrote:
On ppc64, the PowerVM hypervisor runs with limited memory and a VCPU
creation during hotplug may fail during kvm_ioctl for KVM_CREATE_VCPU,
leading to termination of guest since errp is set to &error_fatal while
calling kvm_init_vcpu. This unexpected behaviour can be avoided by
pre-creating vcpu and parking it on success or return error otherwise.
This enables graceful error delivery for any vcpu hotplug failures while
the guest can keep running.
So this puts in on the park list so when kvm_init_vcpu() later runs it
will just take it off the park list instead of issuing another
KVM_CREATE_VCPU ioctl.
And kvm_init_vcpu() runs in the vcpu thread function, which does not
have a good way to indicate failure to the caller.
I'm don't know a lot about this part of qemu but it seems like a good
idea to move fail-able initialisation out of the vcpu thread in that
case. So the general idea seems good to me.
Yeh ..
Based on api refactoring to create/park vcpus introduced in 1/8 of patch series:
https://lore.kernel.org/qemu-devel/20240312020000.12992-2-salil.mehta@huawei.com/
So from this series AFAIKS you're just using kvm_create / kvm_park
routines? You could easily pull that patch 1 out ahead of that larger
series if progress is slow on it, it's a decent cleanup by itself by
the looks.
Yeh, patch 1 of that series is only we need but the author mentioned on
the list that he is about to post next version soon.
Tested OK by repeatedly doing a hotplug/unplug of vcpus as below:
#virsh setvcpus hotplug 40
#virsh setvcpus hotplug 70
error: internal error: unable to execute QEMU command 'device_add':
kvmppc_cpu_realize: vcpu hotplug failed with -12
Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Suggested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off by: Harsh Prateek Bora <harshpb@linux.ibm.com>
---
---
target/ppc/kvm.c | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 8231feb2d4..c887f6dfa0 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -48,6 +48,8 @@
#include "qemu/mmap-alloc.h"
#include "elf.h"
#include "sysemu/kvm_int.h"
+#include "sysemu/kvm.h"
+#include "hw/core/accel-cpu.h"
#define PROC_DEVTREE_CPU "/proc/device-tree/cpus/"
@@ -2339,6 +2341,43 @@ static void alter_insns(uint64_t *word, uint64_t flags, bool on)
}
}
+static int max_cpu_index = 0;
+
+static bool kvmppc_cpu_realize(CPUState *cs, Error **errp)
+{
+ int ret;
+
+ cs->cpu_index = max_cpu_index++;
+
+ POWERPC_CPU(cs)->vcpu_id = cs->cpu_index;
So you're overriding the cpu_get_free_index() allocator here.
And you need to because vcpu_id needs to be assigned before
the KVM create, I guess.
Yes ..
I guess it works. I would add a comment like s390x has.
Not sure which comment you were referring to but with exporting
cpu_get_free_index as suggested later, not sure if we still need any
comment.
Yeah that's true.
+
+ if (cs->parent_obj.hotplugged) {
Can _all_ kvm cpu creation go via this path? Why just limit it to
hotplugged?
For the initial bootup, we actually want to abort if the requested vCPUs
cant be allocated so that user can retry until the requested vCPUs are
allocated. For hotplug failure, bringing down entire guest isn't fair,
hence the fix.
But you could make the error handling depend on hotplugged, no?
Perhaps put that error handling decision in common code so policy
is the same for all targets and back ends.
Hmm, I think just setting errp appropriately would suffice for both
cases as existing behaviour takes care of the rest of handling.
Something like below:
+static bool kvmppc_cpu_realize(CPUState *cs, Error **errp)
+{
+ int ret;
+ const char *vcpu_str = (cs->parent_obj.hotplugged == true) ?
+ "hotplug" : "create";
+ cs->cpu_index = cpu_get_free_index();
+
+ POWERPC_CPU(cs)->vcpu_id = cs->cpu_index;
+
+ /* create and park to fail gracefully in case vcpu hotplug fails */
+ ret = kvm_create_and_park_vcpu(cs);
+ if (ret) {
+ error_setg(errp, "%s: vcpu %s failed with %d",
+ __func__, vcpu_str, ret);
+ return false;
+ }
+ return true;
+}
[...]
+}
+
static void kvmppc_host_cpu_class_init(ObjectClass *oc, void *data)
{
PowerPCCPUClass *pcc = POWERPC_CPU_CLASS(oc);
@@ -2963,4 +3002,7 @@ bool kvm_arch_cpu_check_are_resettable(void)
void kvm_arch_accel_class_init(ObjectClass *oc)
{
+ AccelClass *ac = ACCEL_CLASS(oc);
+ ac->cpu_common_realize = kvmppc_cpu_realize;
+ ac->cpu_common_unrealize = kvmppc_cpu_unrealize;
}
One other thing I noticed -- cpu_common_realize seems to be for
core code and cpu_target_realize for targets. Should we be
using the latter here? If not, a comment would be warranted and
probably also a comment in accel_cpu_common_realize().
Well, I looked at that initially but looks like accel_cpu itself (which
contains vector for cpu_target_realize) doesnt get initialized in case
of kvm on ppc for some reason and therefore wouldnt work. I think
this change is generic enough for cpu_common_realize as well (now having
similar behaviour for initial vcpus also), so I guess comment may not
really be needed, but let me know if still need any comment.
Else, I can post next version with above proposed diff.
regards,
Harsh
Thanks,
Nick