Re: [PATCH] arm64/kvm: Add generic v8 KVM target

["Suzuki K. Poulose" <Suzuki.Poulose@arm.com>]

On 23/06/15 13:39, Christoffer Dall wrote:
> On Mon, Jun 22, 2015 at 09:44:48AM +0100, Peter Maydell wrote:
>> On 17 June 2015 at 10:00, Suzuki K. Poulose <suzuki.poulose@arm.com> wrote:
>>> From: "Suzuki K. Poulose" <suzuki.poulose@arm.com>
>>>
>>> This patch adds a generic ARM v8 KVM target cpu type for use
>>> by the new CPUs which eventualy ends up using the common sys_reg
>>> table. For backward compatibility the existing targets have been
>>> preserved. Any new target CPU that can be covered by generic v8
>>> sys_reg tables should make use of the new generic target.
>>
>> How do you intend this to work for cross-host migration?
>> Is the idea that the kernel guarantees that "generic" looks
>> 100% the same to the guest regardless of host hardware? I'm
>> not sure that can be made to work, given impdef differences
>> in ID register values, bp/wp registers, and so on.
>>
>> Given that, it seems to me that we still need to provide
>> KVM_ARM_TARGET_$THISCPU defines so userspace can request
>> a specific guest CPU flavour; so what does this patch
>> provide that isn't already provided by just having userspace
>> query for the "preferred" CPU type as it does already?
>>
> I'm guessing the intention is to avoid having to add code in the kernel
> to support KVM on a new CPU where nothing else needs to be done to
> support KVM on that system.
Yes, thats the *only* motivation behind the patch and doesn't address
the migration issue. May be we can create a dummy set of values for
the ID registers, which doesn't provide any 'special functionality'
so that it is safe to be migrated across any host ?

>
> Wrt. migration, I was also wondering about this.  Would the differences
> in the CPU architecture be detected when feeding back the invariant
> sysregs from userspace on VM restore?
>
> -Christoffer
>

Suzuki