Subject: Re: [PATCH v3 07/12] KVM: arm64: selftests: Add support to get the vcpuid from MPIDR_EL1

From: Oliver Upton <[email-masked]>
Date: Wed, 1 Sep 2021 23:48:12 +0000

On Wed, Sep 01, 2021 at 09:14:07PM +0000, Raghavendra Rao Ananta wrote:
> At times, such as when in the interrupt handler, the guest wants to
> get the vCPU-id that it's running on. As a result, introduce
> get_vcpuid() that parses the MPIDR_EL1 and returns the vcpuid to the
> requested caller.
> 
> Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
> ---
>  .../selftests/kvm/include/aarch64/processor.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/include/aarch64/processor.h b/tools/testing/selftests/kvm/include/aarch64/processor.h
> index c35bb7b8e870..8b372cd427da 100644
> --- a/tools/testing/selftests/kvm/include/aarch64/processor.h
> +++ b/tools/testing/selftests/kvm/include/aarch64/processor.h
> @@ -251,4 +251,23 @@ static inline void local_irq_disable(void)
>  	asm volatile("msr daifset, #3" : : : "memory");
>  }
>  
> +#define MPIDR_LEVEL_BITS 8
> +#define MPIDR_LEVEL_SHIFT(level) (MPIDR_LEVEL_BITS * level)
> +#define MPIDR_LEVEL_MASK ((1 << MPIDR_LEVEL_BITS) - 1)
> +#define MPIDR_AFFINITY_LEVEL(mpidr, level) \
> +	((mpidr >> MPIDR_LEVEL_SHIFT(level)) & MPIDR_LEVEL_MASK)
> +
> +static inline uint32_t get_vcpuid(void)
> +{
> +	uint32_t vcpuid = 0;
> +	uint64_t mpidr = read_sysreg(mpidr_el1);
> +
> +	/* KVM limits only 16 vCPUs at level 0 */
> +	vcpuid = mpidr & 0x0f;
> +	vcpuid |= MPIDR_AFFINITY_LEVEL(mpidr, 1) << 4;
> +	vcpuid |= MPIDR_AFFINITY_LEVEL(mpidr, 2) << 12;
> +
> +	return vcpuid;
> +}

Are we guaranteed that KVM will always compose vCPU IDs the same way? I
do not believe this is guaranteed ABI.

For the base case, you could pass the vCPU ID as an arg to the guest
function.

I do agree that finding the vCPU ID is a bit more challenging in an
interrupt context. Maybe use a ucall to ask userspace? But of course,
every test implements its own run loop, so its yet another case that
tests need to handle.

Or, you could allocate an array at runtime of length KVM_CAP_MAX_VCPUS
(use the KVM_CHECK_EXTENSION ioctl to get the value). Once all vCPUs are
instantiated, iterate over them from userspace to populate the {MPIDR,
VCPU_ID} map. You'd need to guarantee that callers initialize the vGIC
*after* adding vCPUs to the guest.

--
Thanks,
Oliver

>  #endif /* SELFTEST_KVM_PROCESSOR_H */
> -- 
> 2.33.0.153.gba50c8fa24-goog
> 

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