linux-kernel - Re: [PATCH v2 3/7] KVM: Drop kvm_reload_remote

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Date:   Mon, 28 Feb 2022 14:05:24 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2 3/7] KVM: Drop kvm_reload_remote_mmus(), open code
 request in x86 users

On Fri, Feb 25, 2022 at 10:22 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Remove the generic kvm_reload_remote_mmus() and open code its
> functionality into the two x86 callers.  x86 is (obviously) the only
> architecture that uses the hook, and is also the only architecture that
> uses KVM_REQ_MMU_RELOAD in a way that's consistent with the name.  That
> will change in a future patch, as x86's usage when zapping a single
> shadow page x86 doesn't actually _need_ to reload all vCPUs' MMUs, only
> MMUs whose root is being zapped actually need to be reloaded.
>
> s390 also uses KVM_REQ_MMU_RELOAD, but for a slightly different purpose.
>
> Drop the generic code in anticipation of implementing s390 and x86 arch
> specific requests, which will allow dropping KVM_REQ_MMU_RELOAD entirely.
>
> Opportunistically reword the x86 TDP MMU comment to avoid making
> references to functions (and requests!) when possible, and to remove the
> rather ambiguous "this".
>
> No functional change intended.
>
> Cc: Ben Gardon <bgardon@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c   | 14 +++++++-------
>  include/linux/kvm_host.h |  1 -
>  virt/kvm/kvm_main.c      |  5 -----
>  3 files changed, 7 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b2c1c4eb6007..32c6d4b33d03 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2353,7 +2353,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
>                  * treats invalid shadow pages as being obsolete.
>                  */
>                 if (!is_obsolete_sp(kvm, sp))
> -                       kvm_reload_remote_mmus(kvm);
> +                       kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
>         }
>
>         if (sp->lpage_disallowed)
> @@ -5639,11 +5639,11 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
>          */
>         kvm->arch.mmu_valid_gen = kvm->arch.mmu_valid_gen ? 0 : 1;
>
> -       /* In order to ensure all threads see this change when
> -        * handling the MMU reload signal, this must happen in the
> -        * same critical section as kvm_reload_remote_mmus, and
> -        * before kvm_zap_obsolete_pages as kvm_zap_obsolete_pages
> -        * could drop the MMU lock and yield.
> +       /*
> +        * In order to ensure all vCPUs drop their soon-to-be invalid roots,
> +        * invalidating TDP MMU roots must be done while holding mmu_lock for
> +        * write and in the same critical section as making the reload request,
> +        * e.g. before kvm_zap_obsolete_pages() could drop mmu_lock and yield.
>          */
>         if (is_tdp_mmu_enabled(kvm))
>                 kvm_tdp_mmu_invalidate_all_roots(kvm);
> @@ -5656,7 +5656,7 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
>          * Note: we need to do this under the protection of mmu_lock,
>          * otherwise, vcpu would purge shadow page but miss tlb flush.
>          */
> -       kvm_reload_remote_mmus(kvm);
> +       kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
>
>         kvm_zap_obsolete_pages(kvm);
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f11039944c08..0aeb47cffd43 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1325,7 +1325,6 @@ int kvm_vcpu_yield_to(struct kvm_vcpu *target);
>  void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu, bool usermode_vcpu_not_eligible);
>
>  void kvm_flush_remote_tlbs(struct kvm *kvm);
> -void kvm_reload_remote_mmus(struct kvm *kvm);
>
>  #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
>  int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 83c57bcc6eb6..66bb1631cb89 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -354,11 +354,6 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
>  EXPORT_SYMBOL_GPL(kvm_flush_remote_tlbs);
>  #endif
>
> -void kvm_reload_remote_mmus(struct kvm *kvm)
> -{
> -       kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);
> -}
> -
>  #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
>  static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
>                                                gfp_t gfp_flags)
> --
> 2.35.1.574.g5d30c73bfb-goog
>