Subject: Re: [PATCH 1/4] KVM: PPC: Book3S HV: Remove support for running HPT guest on RPT host without mixed mode support

From: Nicholas Piggin <[email-masked]>
Date: Wed, 20 Jan 2021 10:05:20 +1000

Excerpts from Fabiano Rosas's message of January 20, 2021 7:07 am:
> Nicholas Piggin <[email-masked]> writes:
> 
>> Excerpts from Fabiano Rosas's message of January 19, 2021 11:46 am:
>>> Resending because the previous got spam-filtered:
>>> 
>>> Nicholas Piggin <[email-masked]> writes:
>>> 
>>>> This reverts much of commit c01015091a770 ("KVM: PPC: Book3S HV: Run HPT
>>>> guests on POWER9 radix hosts"), which was required to run HPT guests on
>>>> RPT hosts on early POWER9 CPUs without support for "mixed mode", which
>>>> meant the host could not run with MMU on while guests were running.
>>>>
>>>> This code has some corner case bugs, e.g., when the guest hits a machine
>>>> check or HMI the primary locks up waiting for secondaries to switch LPCR
>>>> to host, which they never do. This could all be fixed in software, but
>>>> most CPUs in production have mixed mode support, and those that don't
>>>> are believed to be all in installations that don't use this capability.
>>>> So simplify things and remove support.
>>> 
>>> With this patch in a DD2.1 machine + indep_threads_mode=N +
>>> disable_radix, QEMU aborts and dumps registers, is that intended?
>>
>> Yes. That configuration is hanging handling MCEs in the guest with some 
>> threads waiting forever to synchronize. Paul suggested it was never a
>> supported configuration so we might just remove it.
>>
> 
> OK, so:
> 
> Tested-by: Fabiano Rosas <[email-masked]>
> 
>>> Could we use the 'no_mixing_hpt_and_radix' logic in check_extension to
>>> advertise only KVM_CAP_PPC_MMU_RADIX to the guest via OV5 so it doesn't
>>> try to run hash?
>>> 
>>> For instance, if I hack QEMU's 'spapr_dt_ov5_platform_support' from
>>> OV5_MMU_BOTH to OV5_MMU_RADIX_300 then it boots succesfuly, but the
>>> guest turns into radix, due to this code in prom_init:
>>> 
>>> prom_parse_mmu_model:
>>> 
>>> case OV5_FEAT(OV5_MMU_RADIX): /* Only Radix */
>>> 	prom_debug("MMU - radix only\n");
>>> 	if (prom_radix_disable) {
>>> 		/*
>>> 		 * If we __have__ to do radix, we're better off ignoring
>>> 		 * the command line rather than not booting.
>>> 		 */
>>> 		prom_printf("WARNING: Ignoring cmdline option disable_radix\n");
>>> 	}
>>> 	support->radix_mmu = true;
>>> 	break;
>>> 
>>> It seems we could explicitly say that the host does not support hash and
>>> that would align with the above code.
>>
>> I'm not sure, sounds like you could, on the other hand these aborts seem 
>> like the prefered failure mode for these kinds of configuration issues, 
>> I don't know what the policy is, is reverting back to radix acceptable?
>>
> 
> Yeah, I couldn't find documentation about why we're reverting back to
> radix. I personally dislike it, but there is already a precedent so I'm
> not sure. A radix guest on a hash host does the same transparent
> conversion AFAICT.
> 
> But despite that, this patch removes support for hash MMU in this
> particular scenario. I don't see why continuing to tell the guest we
> support hash.
> 
> Anyway, here's a patch if you decide to go that way (tested w/ DD2.1 &
> 2.3 machines):

Thanks, I don't mind it, have to see if the maintainer will take it :)

You could add a small changelog / SOB and I could putit after my patch?

> 
> ---
> diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
> index 0a056c64c317b..53743555676d6 100644
> --- a/arch/powerpc/include/asm/kvm_ppc.h
> +++ b/arch/powerpc/include/asm/kvm_ppc.h
> @@ -314,6 +314,7 @@ struct kvmppc_ops {
>  			      int size);
>  	int (*enable_svm)(struct kvm *kvm);
>  	int (*svm_off)(struct kvm *kvm);
> +	bool (*hash_possible)(void);
>  };
>  
>  extern struct kvmppc_ops *kvmppc_hv_ops;
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 6f612d240392f..2d1e8aba22b85 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -5599,6 +5599,15 @@ static int kvmhv_svm_off(struct kvm *kvm)
>  	return ret;
>  }
>  
> +static bool kvmppc_hash_possible(void)
> +{
> +	if (radix_enabled() && no_mixing_hpt_and_radix)
> +		return false;
> +
> +	return cpu_has_feature(CPU_FTR_ARCH_300) &&
> +		cpu_has_feature(CPU_FTR_HVMODE);
> +}

Just be careful, it's hash_v3 specifically. Either make this return true 
for arch < 300 add the ARCH_300 check in the ioctl, or rename to include
v3.

> +
>  static struct kvmppc_ops kvm_ops_hv = {
>  	.get_sregs = kvm_arch_vcpu_ioctl_get_sregs_hv,
>  	.set_sregs = kvm_arch_vcpu_ioctl_set_sregs_hv,
> @@ -5642,6 +5651,7 @@ static struct kvmppc_ops kvm_ops_hv = {
>  	.store_to_eaddr = kvmhv_store_to_eaddr,
>  	.enable_svm = kvmhv_enable_svm,
>  	.svm_off = kvmhv_svm_off,
> +	.hash_possible = kvmppc_hash_possible,
>  };
>  

How about adding an op which can check extensions? It could return false
if it wasn't checked and so default to the generic checks in 
kvm_vm_ioctl_check_extension, and take a pointer to 'r' to set.

>  static int kvm_init_subcore_bitmap(void)
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index cf52d26f49cd7..99ced6c570e74 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -611,8 +611,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  		r = !!(hv_enabled && radix_enabled());
>  		break;
>  	case KVM_CAP_PPC_MMU_HASH_V3:
> -		r = !!(hv_enabled && cpu_has_feature(CPU_FTR_ARCH_300) &&
> -		       cpu_has_feature(CPU_FTR_HVMODE));
> +		r = !!(hv_enabled && kvmppc_hv_ops->hash_possible());
>  		break;
>  	case KVM_CAP_PPC_NESTED_HV:
>  		r = !!(hv_enabled && kvmppc_hv_ops->enable_nested &&

Thanks,
Nick