linux-kernel - Re: [PATCH 1/4] KVM: delete .change

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Message-ID: <87h6gcaxip.fsf@mail.lhotse>
Date: Mon, 08 Apr 2024 21:45:18 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Nicholas Piggin
 <npiggin@...il.com>, Anup Patel <anup@...infault.org>, Atish Patra
 <atishp@...shpatra.org>, Sean Christopherson <seanjc@...gle.com>, Andrew
 Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
 linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback

Paolo Bonzini <pbonzini@...hat.com> writes:
> The .change_pte() MMU notifier callback was intended as an
> optimization. The original point of it was that KSM could tell KVM to flip
> its secondary PTE to a new location without having to first zap it. At
> the time there was also an .invalidate_page() callback; both of them were
> *not* bracketed by calls to mmu_notifier_invalidate_range_{start,end}(),
> and .invalidate_page() also doubled as a fallback implementation of
> .change_pte().
>
> Later on, however, both callbacks were changed to occur within an
> invalidate_range_start/end() block.
>
> In the case of .change_pte(), commit 6bdb913f0a70 ("mm: wrap calls to
> set_pte_at_notify with invalidate_range_start and invalidate_range_end",
> 2012-10-09) did so to remove the fallback from .invalidate_page() to
> .change_pte() and allow sleepable .invalidate_page() hooks.
>
> This however made KVM's usage of the .change_pte() callback completely
> moot, because KVM unmaps the sPTEs during .invalidate_range_start()
> and therefore .change_pte() has no hope of finding a sPTE to change.
> Drop the generic KVM code that dispatches to kvm_set_spte_gfn(), as
> well as all the architecture specific implementations.
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/arm64/kvm/mmu.c                  | 34 -----------------
>  arch/loongarch/include/asm/kvm_host.h |  1 -
>  arch/loongarch/kvm/mmu.c              | 32 ----------------
>  arch/mips/kvm/mmu.c                   | 30 ---------------
>  arch/powerpc/include/asm/kvm_ppc.h    |  1 -
>  arch/powerpc/kvm/book3s.c             |  5 ---
>  arch/powerpc/kvm/book3s.h             |  1 -
>  arch/powerpc/kvm/book3s_64_mmu_hv.c   | 12 ------
>  arch/powerpc/kvm/book3s_hv.c          |  1 -
>  arch/powerpc/kvm/book3s_pr.c          |  7 ----
>  arch/powerpc/kvm/e500_mmu_host.c      |  6 ---

LGTM.

Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)

cheers