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

On Thu, Nov 01, 2018 at 10:18 AM +0100, "Daniel P. Berrangé" <berrange(a)redhat.com&gt; wrote: > 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 that happens only because of udev (see Michal’s answer). > , 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. I’m not sure I understand you correctly, but this patch doesn’t use the VM/QEMU capabilities cache (that would be 'struct _virQEMUCaps') for the caching. Instead it uses 'struct _virQEMUCapsCachePriv' and this struct is initialized only once when initializing the QEMU driver. This should be pretty similar to your proposal with the global static variable.