Subject: Re: [v3 02/15] KVM: PPC: Book3S HV: Cleanup updates for kvm vcpu MMCR

From: Michael Ellerman <[email-masked]>
Date: Wed, 22 Jul 2020 14:37:35 +1000

Athira Rajeev <[email-masked]> writes:
>> On 21-Jul-2020, at 9:24 AM, Paul Mackerras <[email-masked]> wrote:
>> On Fri, Jul 17, 2020 at 10:38:14AM -0400, Athira Rajeev wrote:
>>> Currently `kvm_vcpu_arch` stores all Monitor Mode Control registers
>>> in a flat array in order: mmcr0, mmcr1, mmcra, mmcr2, mmcrs
>>> Split this to give mmcra and mmcrs its own entries in vcpu and
>>> use a flat array for mmcr0 to mmcr2. This patch implements this
>>> cleanup to make code easier to read.
>> 
>> Changing the way KVM stores these values internally is fine, but
>> changing the user ABI is not.  This part:
>> 
>>> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
>>> index 264e266..e55d847 100644
>>> --- a/arch/powerpc/include/uapi/asm/kvm.h
>>> +++ b/arch/powerpc/include/uapi/asm/kvm.h
>>> @@ -510,8 +510,8 @@ struct kvm_ppc_cpu_char {
>>> 
>>> #define KVM_REG_PPC_MMCR0	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x10)
>>> #define KVM_REG_PPC_MMCR1	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x11)
>>> -#define KVM_REG_PPC_MMCRA	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x12)
>>> -#define KVM_REG_PPC_MMCR2	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x13)
>>> +#define KVM_REG_PPC_MMCR2	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x12)
>>> +#define KVM_REG_PPC_MMCRA	(KVM_REG_PPC | KVM_REG_SIZE_U64 | 0x13)
>> 
>> means that existing userspace programs that used to work would now be
>> broken.  That is not acceptable (breaking the user ABI is only ever
>> acceptable with a very compelling reason).  So NAK to this part of the
>> patch.
>
> Hi Paul
>
> Thanks for checking the patch. I understood your point on user ABI breakage that this particular change can cause.
> I will retain original KVM_REG_PPC_MMCRA and KVM_REG_PPC_MMCR2 order in `kvm.h`
> And with that, additionally I will need below change ( on top of current patch ) for my clean up updates for kvm cpu MMCR to work,
> Because now mmcra and mmcrs will have its own entries in vcpu and is not part of the mmcr[] array
> Please suggest if this looks good

I did the same patch I think in my testing branch, it's here:

https://github.com/linuxppc/linux/commit/daea78154eff1b7e2f36be05a8f95feb5a588912

Can you please check that matches what you sent.

cheers

> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 3f90eee261fc..b10bb404f0d5 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1679,10 +1679,13 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
>         case KVM_REG_PPC_UAMOR:
>                 *val = get_reg_val(id, vcpu->arch.uamor);
>                 break;
> -       case KVM_REG_PPC_MMCR0 ... KVM_REG_PPC_MMCR2:
> +       case KVM_REG_PPC_MMCR0 ... KVM_REG_PPC_MMCR1:
>                 i = id - KVM_REG_PPC_MMCR0;
>                 *val = get_reg_val(id, vcpu->arch.mmcr[i]);
>                 break;
> +       case KVM_REG_PPC_MMCR2:
> +               *val = get_reg_val(id, vcpu->arch.mmcr[2]);
> +               break;
>         case KVM_REG_PPC_MMCRA:
>                 *val = get_reg_val(id, vcpu->arch.mmcra);
>                 break;
> @@ -1906,10 +1909,13 @@ static int kvmppc_set_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
>         case KVM_REG_PPC_UAMOR:
>                 vcpu->arch.uamor = set_reg_val(id, *val);
>                 break;
> -       case KVM_REG_PPC_MMCR0 ... KVM_REG_PPC_MMCR2:
> +       case KVM_REG_PPC_MMCR0 ... KVM_REG_PPC_MMCR1:
>                 i = id - KVM_REG_PPC_MMCR0;
>                 vcpu->arch.mmcr[i] = set_reg_val(id, *val);
>                 break;
> +       case KVM_REG_PPC_MMCR2:
> +               vcpu->arch.mmcr[2] = set_reg_val(id, *val);
> +               break;
>         case KVM_REG_PPC_MMCRA:
>                 vcpu->arch.mmcra = set_reg_val(id, *val);
>                 break;
> —
>
>
>> 
>> Regards,
>> Paul.