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

[Marc Hartmayer]

On Tue, Nov 20, 2018 at 11:00 AM +0100, Marc Hartmayer <mhartmay at linux.ibm.com> wrote:
> On Thu, Nov 01, 2018 at 04:37 PM +0100, Michal Privoznik <mprivozn at redhat.com> wrote:
>> 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 at redhat.com>
>>>>>>>> Signed-off-by: Marc Hartmayer <mhartmay at 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.
>
> Are there any other objections against this patch? Because even with the
> ctime bug in udev it’s performs _much_ better than before.

Polite ping :)

[…snip]

--
Kind regards / Beste Grüße
   Marc Hartmayer

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