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

[Eduardo Habkost]

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.

There's a table with Python and GCC versions at
[https://wiki.qemu.org/Supported_Build_Platforms].
Maybe it could include kernel version information as well.

-- 
Eduardo