Re: [PATCH] qga: Correct loop count in qmp_guest_get_vcpus()

[Lin Ma <lma@suse.de>]

On 2020-11-19 14:46, Marc-André Lureau wrote:
> Hi
> 
> On Thu, Nov 19, 2020 at 12:48 PM Lin Ma <lma@suse.com> wrote:
> 
>> The guest-get-vcpus returns incorrect vcpu info in case we hotunplug
>> vcpus(not
>> the last one).
>> e.g.:
>> A VM has 4 VCPUs: cpu0 + 3 hotunpluggable online vcpus(cpu1, cpu2 and
>> cpu3).
>> Hotunplug cpu2,  Now only cpu0, cpu1 and cpu3 are present & online.
>> 
>> ./qmp-shell /tmp/qmp-monitor.sock
>> (QEMU) query-hotpluggable-cpus
>> {"return": [
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 3}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/peripheral/cpu3", "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 2}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/peripheral/cpu2", "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 1}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/peripheral/cpu1", "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 0}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/unattached/device[0]", "type": 
>> "host-x86_64-cpu"}
>> ]}
>> 
>> (QEMU) device_del id=cpu2
>> {"return": {}}
>> 
>> (QEMU) query-hotpluggable-cpus
>> {"return": [
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 3}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/peripheral/cpu3", "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 2}, 
>> "vcpus-count": 1,
>>  "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 1}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/peripheral/cpu1", "type": "host-x86_64-cpu"},
>> {"props": {"core-id": 0, "thread-id": 0, "socket-id": 0}, 
>> "vcpus-count": 1,
>>  "qom-path": "/machine/unattached/device[0]", "type": 
>> "host-x86_64-cpu"}
>> ]}
>> 
>> Before:
>> ./qmp-shell -N /tmp/qmp-ga.sock
>> Welcome to the QMP low-level shell!
>> Connected
>> (QEMU) guest-get-vcpus
>> {"return": [
>> {"online": true, "can-offline": false, "logical-id": 0},
>> {"online": true, "can-offline": true, "logical-id": 1}]}
>> 
>> After:
>> ./qmp-shell -N /tmp/qmp-ga.sock
>> Welcome to the QMP low-level shell!
>> Connected
>> (QEMU) guest-get-vcpus
>> {"execute":"guest-get-vcpus"}
>> {"return": [
>> {"online": true, "can-offline": false, "logical-id": 0},
>> {"online": true, "can-offline": true, "logical-id": 1},
>> {"online": true, "can-offline": true, "logical-id": 3}]}
>> 
>> Signed-off-by: Lin Ma <lma@suse.com>
>> ---
>>  qga/commands-posix.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>> 
>> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
>> index 3bffee99d4..accc893373 100644
>> --- a/qga/commands-posix.c
>> +++ b/qga/commands-posix.c
>> @@ -2182,15 +2182,15 @@ GuestLogicalProcessorList
>> *qmp_guest_get_vcpus(Error **errp)
>>  {
>>      int64_t current;
>>      GuestLogicalProcessorList *head, **link;
>> -    long sc_max;
>> +    long max_loop_count;
>>      Error *local_err = NULL;
>> 
>>      current = 0;
>>      head = NULL;
>>      link = &head;
>> -    sc_max = SYSCONF_EXACT(_SC_NPROCESSORS_CONF, &local_err);
>> +    max_loop_count = SYSCONF_EXACT(_SC_NPROCESSORS_CONF, &local_err);
>> 
>> -    while (local_err == NULL && current < sc_max) {
>> +    while (local_err == NULL && current < max_loop_count) {
>>          GuestLogicalProcessor *vcpu;
>>          GuestLogicalProcessorList *entry;
>>          int64_t id = current++;
>> @@ -2206,6 +2206,8 @@ GuestLogicalProcessorList 
>> *qmp_guest_get_vcpus(Error
>> **errp)
>>              entry->value = vcpu;
>>              *link = entry;
>>              link = &entry->next;
>> +        } else {
>> +            max_loop_count += 1;
>> 
> 
> This looks like a recipe for infinite loop on error.
Emm...It is possible.
> 
> Shouldn't we loop over all the /sys/devices/system/cpu/cpu#/ instead?
Originally I'd like to use the function fnmatch to handle pattern cpu# 
to
loop over all of the /sys/devices/system/cpu/cpu#/, But it introduces 
the
header file fnmatch.h and make things complicated a little.

> 
> (possibly parse /sys/devices/system/cpu/present, but I doubt it's 
> necessary)
IMO the 'present' won't help.

I'm about to post the V2, I made tiny change in the V2, Please help to 
review.

BTW, The local_err will be set in case of error, right? It could avoid 
infinite loop.

Thanks a lot,
Lin