Re: [PATCH v6 13/19] i386: prefer system KVM_GET_SUPPORTED_HV_CPUID ioctl over vCPU's one

[Daniel P . Berrangé]

On Wed, May 26, 2021 at 12:46:25PM -0400, Eduardo Habkost wrote:
> On Mon, May 24, 2021 at 02:08:26PM +0200, Vitaly Kuznetsov wrote:
> [...]
> > >> @@ -980,7 +989,7 @@ static struct kvm_cpuid2 
> > >> *get_supported_hv_cpuid(CPUState *cs)
> > >>       * information early, just check for the capability and set the bit
> > >>       * manually.
> > >>       */
> > >> -    if (kvm_check_extension(cs->kvm_state,
> > >> +    if (!do_sys_ioctl && kvm_check_extension(cs->kvm_state,
> > >>                              KVM_CAP_HYPERV_ENLIGHTENED_VMCS) > 0) {
> > >
> > > Oh, this conditional replaces the comment I suggested in patch
> > > 10/19.  It makes it obvious that the hack can be deleted if we
> > > remove support for the VCPU ioctl.
> > >
> > > So, when exactly will we be able to delete the VCPU ioctl code
> > > and support only the system ioctl?
> > 
> > When QEMU drops support for kernels < 5.11? Note, current RHEL8 already
> > supports system version so we're talking about upstream kernels/Ubuntu
> > LTS/... 
> > 
> > I remember there was a list of supported kernels for QEMU somewhere but
> > don't seem to be able to find it quickly, could you maybe point me in
> > the right direction?
> 
> The KVM-specific kernel requirement is documented here:
> https://qemu-project.gitlab.io/qemu/system/target-i386.html?highlight=kvm#os-requirements
> 
> I took a while to find it.  Maybe we should have a more visible
> "runtime requirements" section in the docs, or it should be
> moved to the supported build platforms section.
> 
> We have a clear policy on supported build platforms
> [https://qemu-project.gitlab.io/qemu/system/build-platforms.html],
> but not a clear policy for KVM kernel dependencies.

While it says "supported build platforms", that was implicitly
assumed to also refer to "runtime platforms". We should just
rename it to "supported-platforms.html" to make it more obvious.

Thus, the minimum KVM kernel version we need follows the same rules.
Look at whatever is the oldest kernel across the distros we target.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|