linux-kernel - Re: [RFC PATCH v11 04/29] KVM: PPC: Drop dead code related to KVM_ARCH_WANT_MMU

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Date:   Wed, 19 Jul 2023 19:34:22 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Alexander Graf <graf@...zon.de>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.linux.dev, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 04/29] KVM: PPC: Drop dead code related to
 KVM_ARCH_WANT_MMU_NOTIFIER

On 7/19/23 01:44, Sean Christopherson wrote:
> Signed-off-by: Sean Christopherson<seanjc@...gle.com>
> ---
>   arch/powerpc/kvm/powerpc.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> index 7197c8256668..5cf9e5e3112a 100644
> --- a/arch/powerpc/kvm/powerpc.c
> +++ b/arch/powerpc/kvm/powerpc.c
> @@ -634,10 +634,11 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	case KVM_CAP_SYNC_MMU:
>   #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
>   		r = hv_enabled;

This could actually be unnecessarily conservative.  Even book3s_pr.c 
knows how to do unmap and set_spte, so it should be able to support 
KVM_CAP_SYNC_MMU.  Alex, Nick, do you remember any of this?  This would 
mean moving KVM_CAP_SYNC_MMU to virt/kvm/kvm_main.c, which is nice.

Paolo

> -#elif defined(KVM_ARCH_WANT_MMU_NOTIFIER)
> -		r = 1;
>   #else
> -		r = 0;
> +#ifndef KVM_ARCH_WANT_MMU_NOTIFIER
> +		BUILD_BUG();
> +#endif
> +		r = 1;
>   #endif
>   		break;
>   #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE