Re: kvm_target, QEMU_KVM_ARM_TARGET_GENERIC_V8 questions

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 4 Jun 2020 at 14:32, Andrew Jones <drjones@redhat.com> wrote:
> On Thu, Jun 04, 2020 at 02:10:08PM +0100, Peter Maydell wrote:
> > These explicit settings are correct, because for these CPUs
> > the kernel does have a "give me what I want in particular"
> > setting (which it will fail on the wrong h/w), and also as
> > back-compat for older kernels that predate the GENERIC_V8
> > define and only recognize the explicit "give me an A53" value.
>
> Actually, I think the failing for the wrong hardware is about all these
> older targets do. I didn't look real closely, but I think all targets
> produce the same result for the guest, which is to pass through the host
> ID registers.

Yes; it's just that originally "specify CPU exactly" was the
only interface, and there wasn't a GENERIC_V8 at all.
I actually suspect that current QEMU will no longer work
on a kernel that's so old that it lacks the GENERIC_V8 and
PREFERRED_TARGET support[*], but we don't have an explicit "we need
at least host kernel version X" requirement that we track, so
it's hard to say for certain. (If we cared enough to test we
could likely delete a bit of back-compat handling code in QEMU.)

[*] in particular I have a feeling that recent changes to the
GIC handling code in the virt board implicitly dropped handling
for ancient kernels

thanks
-- PMM