Re: kvm_target, QEMU_KVM_ARM_TARGET_GENERIC_V8 questions

[Peter Maydell]

[added kvm-arm to the cc list; the kernel folks tend to hang out
there, not on qemu-devel, so KVM related questions are usually
worth raising there as well.]

On Thu, 4 Jun 2020 at 13:55, Leif Lindholm <leif@nuviainc.com> wrote:
> However, while looking at this, I noticed aarch64_a72_initfn doesn't
> initialise kvm_target at all.

Yep. The kernel doesn't support "give me a CPU that looks like
a Cortex-A72".

> So, then I decided to actually test things, and found that
> (with -enable-kvm):
> - on Cortex-A53 hardware
>   - "max" kvm_target gets initialized to 4 (KVM_ARM_TARGET_CORTEX_A53)
>     by kvm_arm_get_host_cpu_features (as returned from the kernel for
>     vm_arm_create_scratch_host_vcpu)
>   - cortex-A72 fails to start with "KVM is not supported for this guest
>     CPU type"
>   (fair enough, it's later than A53)

Untested, but I assume that -cpu cortex-a53 works on the A53...

> - on Cortex-A72 hardware
>   - "max" kvm_target gets initialized to 5 (KVM_ARM_TARGET_GENERIC_V8)
>     by kvm_arm_get_host_cpu_features
>   - "cortex-A72" fails to start (umm...)

...and fails on the A72 host.

> However ... if I haven't managed to confuse myself somewhere in here
> (which is completely possible), would it be OK if I submitted a set of
> patches that:
> - add a QEMU_KVM_ARM_TARGET_GENERIC_V8 to match the kernel one
> - set kvm_target for Cortex-A72 to QEMU_KVM_ARM_TARGET_GENERIC_V8

This would be wrong -- it would mean that you could tell QEMU "give
me a guest CPU that's a Cortex-A72" and it would not error on
non-A72 hardware but not actually give a guest CPU that looks
like a Cortex-A72.

 * If what you want is "give me something that works" then that's
   -cpu host or -cpu max.

 * If what you want is "give me something that's always this kind of
   CPU regardless of the host hardware" then that's a lot of kernel
   dev work nobody's been enthusiastic enough to undertake yet
   (notably the "what do we do about CPU errata workarounds" question
   would need to be solved...)

 * If what you want is "allow me to say '-cpu cortex-a72' and have
   it work on an A72 host and not anywhere else" then I guess we could
   implement that on the QEMU side by querying the MIDR and checking
   whether it was what we expected.

>   - alternatively drop the explicit settings for A57/A53

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.

> - drop the call from aarch64_max_initfn to aarch64_a57_initfn, and
>   copy the relevant bits into the former for the !kvm case

Not sure why you care about this -- it's an implementation
detail of the TCG handling of the 'max' CPU. There's an argument
for disentangling TCG 'max' so it's not literally implemented
as "a57 plus newer stuff", granted.

thanks
-- PMM