Re: [PATCH] target/arm: do not use cc->do_interrupt for KVM directly

[Claudio Fontana]

On 12/7/20 7:07 PM, Peter Maydell wrote:
> On Mon, 7 Dec 2020 at 17:49, Eduardo Habkost <ehabkost@redhat.com> wrote:
>> arm_v7m_class_init() (used by cortex-* CPU models) overrides it.
>> Those CPU models as "TCG CPUs" in the code, but I don't see what
>> makes them TCG-specific.
> 
> They're TCG specific because the Arm Virtualization Extension
> is for A-profile only and only supports virtualization of
> A-profile CPUs. You can't accelerate an M-profile CPU with it.
> (Similarly, you can't use the Virtualization Extension to
> accelerate a pre-v7 CPU, which is why CPUs like the arm1176
> are also TCG-only, and you can't use it to accelerate a CPU
> which has TrustZone enabled.)
> 
> (M-profile CPUs override cc->do_interrupt() because their
> exception and interrupt handling logic is totally different
> to A-profile.)
> 
>> What exactly is the expected behavior
>> if using, e.g., "-cpu cortex-m33 -accel kvm"?
> 
> It ought to print a useful error message telling you
> that that CPU type isn't compatible with KVM.
> 
> As it happens, you get an assertion failure for cortex-m33:
> 
> $ ./build/all/qemu-system-aarch64 -accel kvm -M mps2-an505 -display none
> qemu-system-aarch64: ../../softmmu/physmem.c:745:
> cpu_address_space_init: Assertion `asidx == 0 || !kvm_enabled()'
> failed.
> Aborted
> 
> because the M33 has TrustZone enabled (which is not compatible
> with KVM) and we don't check that before we hit the assertion.
> We should fix that :-)
> 
> If you try it with a non-TrustZone M-profile core like the M3 then
> you do get a better error message:
> 
> $ ./build/all/qemu-system-aarch64 -accel kvm -M mps2-an385 -display none
> qemu-system-aarch64: KVM is not supported for this guest CPU type
> qemu-system-aarch64: kvm_init_vcpu: kvm_arch_init_vcpu failed (0):
> Invalid argument
> 
> If you try this with the "virt" board then you'll run into the
> virt board's own sanity checking of CPU types instead:
> 
> $ ./build/all/qemu-system-aarch64 -accel kvm -M virt -cpu cortex-m33
> -display none
> qemu-system-aarch64: mach-virt: CPU type cortex-m33-arm-cpu not supported
> 
> All of that said, I think I agree with you -- we have this
> indirect mechanism for invoking class methods on the CPU
> object, why is it necessary for this KVM-specific code to
> call the implementation function directly?
> 
> thanks
> -- PMM
> 

Hi Peter,

the initial attempt there was to mark do_interrupt as a TCG-only operation,
to move to a separate tcg_ops structure in CPUClass like for the others,

but countrary to the other ops I noticed that ARM is the only target that is 
calling cc->do_interrupt() for KVM too.

I might have to either leave do_interrupt() out of the refactoring, or find a 
way to avoid accessing cc->do_interrupt on KVM.

Thanks,

Claudio