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



On Thu, Nov 01, 2018 at 09:31:50AM +0100, Martin Kletzander wrote:
> On Tue, Oct 30, 2018 at 03:07:59PM +0000, Daniel P. Berrangé wrote:
> > On Tue, Oct 30, 2018 at 03:45:36PM +0100, Michal Privoznik wrote:
> > > On 10/30/2018 02:46 PM, Michal Privoznik wrote:
> > > >
> > > > It is kernel problem. In my testing, the moment I call:
> > > >
> > > >  ioctl(kvm, KVM_CREATE_VM, 0);
> > > 
> > > Okay, I have to retract this claim. 'udevadm monitor' shows some events:
> > > 
> > > KERNEL[3631.129645] change   /devices/virtual/misc/kvm (misc)
> > > UDEV  [3631.130816] change   /devices/virtual/misc/kvm (misc)
> > > 
> > > and stopping udevd leaves all three times untouched. So it is udev after
> > > all. I just don't know how to find the rule that is causing the issue.
> > > Anyway, as for this patch, I think we can merge it in the end, can't we?
> > 
> > Not really. Udev is in use everywhere, so this behaviour makes the
> > patch useless in practice, even though it is technically right in
> > theory :-(
> > 
> 
> Does it?  With this behaviour we still do the "expensive" work after any machine
> has started.  But for one machine starting it still has the effect of running it
> only once.  And we *need* to run it for each machine unless we also cache the
> result per (at least) user:group of that machine as every machine can run under
> different user:group.

Yes, with the current patch it still rechecks it for each VM startup, but
I was going to suggest the caching of this is simply put in a global static
variable as there's no reason to record this device permissions state in
the per VM caps cache.


Regards,
Daniel
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