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Re: [libvirt] [PATCH v2] qemu: Introduce caching whether /dev/kvm is accessible



On 10/30/2018 01:55 PM, Daniel P. Berrangé wrote:
> On Tue, Oct 30, 2018 at 10:32:08AM +0000, Daniel P. Berrangé wrote:
>> On Tue, Oct 30, 2018 at 11:08:45AM +0100, Michal Privoznik wrote:
>>> On 10/30/2018 10:35 AM, Daniel P. Berrangé wrote:
>>>> On Tue, Oct 30, 2018 at 09:13:50AM +0100, Michal Privoznik wrote:
>>>>> On 10/29/2018 06:34 PM, Marc Hartmayer wrote:
>>>>>> Introduce caching whether /dev/kvm is usable as the QEMU user:QEMU
>>>>>> group. This reduces the overhead of the QEMU capabilities cache
>>>>>> lookup. Before this patch there were many fork() calls used for
>>>>>> checking whether /dev/kvm is accessible. Now we store the result
>>>>>> whether /dev/kvm is accessible or not and we only need to re-run the
>>>>>> virFileAccessibleAs check if the ctime of /dev/kvm has changed.
>>>>>>
>>>>>> Suggested-by: Daniel P. Berrangé <berrange redhat com>
>>>>>> Signed-off-by: Marc Hartmayer <mhartmay linux ibm com>
>>>>>> ---
>>>>>>  src/qemu/qemu_capabilities.c | 54 ++++++++++++++++++++++++++++++++++--
>>>>>>  1 file changed, 52 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
>>>>>> index e228f52ec0bb..85516954149b 100644
>>>>>> --- a/src/qemu/qemu_capabilities.c
>>>>>> +++ b/src/qemu/qemu_capabilities.c
>>>>>> @@ -3238,6 +3238,10 @@ struct _virQEMUCapsCachePriv {
>>>>>>      virArch hostArch;
>>>>>>      unsigned int microcodeVersion;
>>>>>>      char *kernelVersion;
>>>>>> +
>>>>>> +    /* cache whether /dev/kvm is usable as runUid:runGuid */
>>>>>> +    virTristateBool kvmUsable;
>>>>>> +    time_t kvmCtime;
>>>>>>  };
>>>>>>  typedef struct _virQEMUCapsCachePriv virQEMUCapsCachePriv;
>>>>>>  typedef virQEMUCapsCachePriv *virQEMUCapsCachePrivPtr;
>>>>>> @@ -3824,6 +3828,52 @@ virQEMUCapsSaveFile(void *data,
>>>>>>  }
>>>>>>  
>>>>>>  
>>>>>> +/* Determine whether '/dev/kvm' is usable as QEMU user:QEMU group. */
>>>>>> +static bool
>>>>>> +virQEMUCapsKVMUsable(virQEMUCapsCachePrivPtr priv)
>>>>>> +{
>>>>>> +    struct stat sb;
>>>>>> +    static const char *kvm_device = "/dev/kvm";
>>>>>> +    virTristateBool value;
>>>>>> +    virTristateBool cached_value = priv->kvmUsable;
>>>>>> +    time_t kvm_ctime;
>>>>>> +    time_t cached_kvm_ctime = priv->kvmCtime;
>>>>>> +
>>>>>> +    if (stat(kvm_device, &sb) < 0) {
>>>>>> +        virReportSystemError(errno,
>>>>>> +                             _("Failed to stat %s"), kvm_device);
>>>>>> +        return false;
>>>>>> +    }
>>>>>> +    kvm_ctime = sb.st_ctime;
>>>>>
>>>>> This doesn't feel right. /dev/kvm ctime is changed every time qemu is
>>>>> started or powered off (try running stat over it before and after a
>>>>> domain is started/shut off). So effectively we will fork more often than
>>>>> we would think. Should we cache inode number instead? Because for all
>>>>> that we care is simply if the file is there.
>>>>
>>>> Urgh, that is a bit strange and not the usual semantics for timestamps :-(
>>>
>>> Indeed.
>>>
>>>>
>>>> We can't stat the inode - the code was explicitly trying to cope with the
>>>> way /dev/kvm can change permissions when udev rules get applied. We would
>>>> have to compare the user, group, permissions mask and even ACL, or a hash
>>>> of those.
>>>
>>> Well, we can use ctime as suggested and post a patch for kernel to fix
>>> ctime behaviour. Until the patch is merged our behaviour would be
>>> suboptimal, but still better than it is now.
>>
>> I guess lets talk to KVM team for their input on this and then decide
>> what todo.
> 
> Hmm, I wonder if it is not actually a kernel problem, but rather something
> in userspace genuinely touching the device in a way that caues these
> timestamps to be updated.
> 
> eg I vaguely recall a udev rule that resets permissions on device nodes
> whenever an FD is closed, which might cause this kind of behaviour

After trying hard to find the rule that mangles the ctime I've found the
following bug in udev:

https://github.com/zippy2/systemd/commit/51915e4e6a64c581da321c25448d80626d8e8408?diff=unified

I've send that as a pull request to systemd. So after all, ctime might
be usable again.

Michal


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