Re: [PATCH 14/29] arm64: KVM: guest one-reg interface

[Marc Zyngier <marc.zyngier@arm.com>]

On 12/03/13 17:31, Christopher Covington wrote:
> Hi Marc,
> 
> On 03/04/2013 10:47 PM, Marc Zyngier wrote:
>> Let userspace play with the guest registers.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>> ---
>>  arch/arm64/kvm/guest.c | 240 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 240 insertions(+)
>>  create mode 100644 arch/arm64/kvm/guest.c
> 
> [...]
> 
>> +int __attribute_const__ kvm_target_cpu(void)
>> +{
>> +	unsigned long implementor = read_cpuid_implementor();
>> +	unsigned long part_number = read_cpuid_part_number();
>> +
>> +	if (implementor != ARM_CPU_IMP_ARM)
>> +		return -EINVAL;
>> +
>> +	switch (part_number) {
>> +	case ARM_CPU_PART_AEM_V8:
>> +	case ARM_CPU_PART_FOUNDATION:
>> +		/* Treat the models just as an A57 for the time being */
>> +	case ARM_CPU_PART_CORTEX_A57:
>> +		return KVM_ARM_TARGET_CORTEX_A57;
>> +	default:
>> +		return -EINVAL;
>> +	}
>> +}
> 
> What is the motivation behind these checks? Why not let any ARMv8 system that
> has EL2 host a virtualized Cortex A57 guest?

The main reason is errata management. How do you deal with errata in the
guest when you hide the underlying host CPU? I don't have an answer to
that. So for the time being, we only allow the guest to see the same CPU
as the host, and require that new CPUs are added to this function.

We went the same way for KVM/ARM.

	M.
-- 
Jazz is not dead. It just smells funny...