[PATCH] KVM: arm64: Clarify vcpu reset behaviour

[Alexandru Elisei]

Hi Marc,

On 4/6/21 1:58 PM, Marc Zyngier wrote:
> Although the KVM_ARM_VCPU_INIT documentation mention that the
> registers are reset to their "initial values", it doesn't
> describe what these values are.
>
> Describe this state explicitly.

This is a good idea.

>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  Documentation/virt/kvm/api.rst | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 38e327d4b479..e2237e4e10ba 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -3115,6 +3115,16 @@ optional features it should have.  This will cause a reset of the cpu
>  registers to their initial values.  If this is not called, KVM_RUN will
>  return ENOEXEC for that vcpu.
>  
> +The initial values are defined as:
> +	- Processor state:
> +		* AArch64: EL1h, D, A, I and F bits set

That value matches the macro VCPU_RESET_STATE_EL1.

> +		* Aarch32: SVC, D, A, I and F bits set
VCPU_RESET_STATE_SVC doesn't have a D bit; according to ARM DDI 0487G.a, page
G1-5965, CPSR doesn't have a D bit either (I might be reading it wrong).
> +	- General Purpose registers, including PC and SP: set to 0

They are zero'ed explicitly in kvm_reset_vcpu().

> +	- FPSIMD/NEON registers: set to 0

I haven't been able to find where they are initialized explicitly, I assume they
aren't. They are zero because the kvm_vcpu struct is allocated via
kmem_cache_zalloc() in kvm_vm_ioctl_create_vcpu(), so this is correct.

> +	- SVE registers: set to 0

They are zero'ed explicitly in kvm_vcpu_reset_sve().

> +	- System registers: Reset to their architecturally defined
> +	  values as for a warm reset to EL1 (resp. SVC)

This is done in kvm_reset_vcpu() -> kvm_reset_sys_regs(), which from what I can
tell does the right thing, but I haven't checked each and every register.

Assuming the D bit for CPSR/SPSR was a typo and with it removed:

Reviewed-by: Alexandru Elisei <alexandru.elisei at arm.com>

Thanks,

Alex

> +
>  Note that because some registers reflect machine topology, all vcpus
>  should be created before this ioctl is invoked.
>