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

From: Madhavan Srinivasan <maddy@linux.ibm.com>
To: Paul Mackerras <paulus@ozlabs.org>,
	Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: ego@linux.vnet.ibm.com, Michael Neuling <mikey@neuling.org>,
	maddy@linux.vnet.ibm.com, kvm@vger.kernel.org,
	kvm-ppc@vger.kernel.org, svaidyan@in.ibm.com, acme@kernel.org,
	jolsa@kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [v3 02/15] KVM: PPC: Book3S HV: Cleanup updates for kvm vcpu MMCR
Date: Wed, 22 Jul 2020 11:33:45 +0530	[thread overview]
Message-ID: <cc45ef85-4442-3377-12dd-d4ca22fd7af5@linux.ibm.com> (raw)
In-Reply-To: <20200722045448.GC3878639@thinks.paulus.ozlabs.org>

On 7/22/20 10:24 AM, Paul Mackerras wrote:
> On Wed, Jul 22, 2020 at 07:39:26AM +0530, Athira Rajeev wrote:
>>
>>> On 21-Jul-2020, at 9:24 AM, Paul Mackerras <paulus@ozlabs.org> 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
> Yes, that looks fine.
>
> By the way, is the new MMCRS register here at all related to the MMCRS
Hi Paul,

We have only split the current array (mmcr[]) and separated it to mmcra 
and mmcrs.
Only new spr that is added is mmcr3 (for Power10).

Maddy

> that there used to be on POWER8, but which wasn't present (as far as I
> know) on POWER9?
>
> Paul.