Re: arm64 disable some feature bit of ID_AA64ISAR0_EL1

[Jiatong Shen]

Thank you very much for the clarification. Although cpu type cannot be changed, another 

registers like aa64isar0, aa64isar1 could be changed through KVM_SET_ONE_REG interface, 

and then could potentially hide some cpu features from host. Am I right?

Another questions is why does arm64 have some invariant system registers? 

thank you very much for the generous support!

Best,

Jiatong Shen

On Wed, May 17, 2023 at 9:13 PM Peter Maydell <peter.maydell@linaro.org> wrote:

On Wed, 17 May 2023 at 14:06, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Wed, 17 May 2023 at 03:23, Jiatong Shen <yshxxsjt715@gmail.com> wrote:
> >
> > Hello community Experts!
> >
> >     I am doing some experiments with arm64 host machine. The host machine is kunpeng 920 5251k. I try to boot a virtual machine with the following command:
> >
> > qemu-system-aarch64 -cpu cortex-a53 -enable-kvm -smp 1 -m 4096 -M virt,gic-version=3
> >
> > Here I have modified the code, and change kvm_target from QEMU_KVM_ARM_TARGET_CORTEX_A53 to 5(generic arm  v8).
>
> This is part of what is confusing you: this is not a correct change.
>
> > when the vm boots, I try to use lscpu, but I still saw some feature flags like atomics exist.
>
> In a KVM VM you will always see in the guest the exact same
> CPU type as the host. There is no support in the kernel for
> showing the guest a different CPU type to what the hardware has.

For clarity, I mean this specifically for the Arm architecture;
x86 is different and will let you expose a different CPU type
to the guest.

-- PMM

--

Best Regards,

Jiatong Shen