[libvirt] [PATCH v2] qemu: Add check for whether KVM nesting is enabled

[Daniel P. Berrangé]

On Tue, Nov 27, 2018 at 02:15:39PM -0500, John Ferlan wrote:
> 
> 
> On 11/27/18 12:05 PM, Andrea Bolognani wrote:
> > On Mon, 2018-11-26 at 18:38 -0500, John Ferlan wrote:
> > [...]
> >> +static bool
> >> +virQEMUCapsKVMIsNested(void)
> >> +{
> >> +    VIR_AUTOFREE(char *) kConfig = NULL;
> >> +
> >> +    /* Intel, AMD, and s390 related checks */
> >> +    if ((kConfig = virKModConfig()) &&
> >> +        (strstr(kConfig, "kvm_intel nested=1") ||
> >> +         strstr(kConfig, "kvm_amd nested=1") ||
> >> +         strstr(kConfig, "kvm nested=1")))
> >> +        return true;
> >> +    return false;
> >> +}
> > 
> > I might be doing it wrong, but I'm pretty sure I've enabled nested
> > virtualization properly on my laptop given that I can successfully
> > run 'modprobe kvm_intel' inside the L1 guest, and yet I get
> > 
> >   # modprobe -c | grep -c nested=1
> >   0
> > 
> > both in the L0 host and the L1 guest, so this check doesn't seem
> > accurate to me.
> > 
> > Oh, wait, I get it now: 'modprobe -c' doesn't dump the *current* host
> > configuration, but the *static* one! So if you enable nested KVM
> > support by doing
> > 
> >   # modprobe -r kvm_intel
> >   # modprobe kvm_intel nested=1
> > 
> > like I did, then the check above will not report it as enabled even
> > though it is; conversely, if you drop the appropriate config snippet
> > in /etc/modprobe.d/ but don't reload the module it will report it as
> > enabled even though it's not!
> 
> Ugh, sigh... Yep, I was thinking primarily the static config option
> since we had helpers to read. Of course that won't be enough. Joy, more
> code to probe... Maybe it is easier to just say - clear your
> capabilities cache if you alter that particular kernel value.

Surely its already easier just to ask the kernel for the live status

$ cat  /sys/module/kvm_intel/parameters/nested
1

likewise for other kmods

Regards,
Daniel
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