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

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Message-ID: <20240412104408.GA27645@willie-the-truck>
Date: Fri, 12 Apr 2024 11:44:09 +0100
From: Will Deacon <will@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	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

On Fri, Apr 05, 2024 at 07:58:12AM -0400, Paolo Bonzini wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index dc04bc767865..ff17849be9f4 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1768,40 +1768,6 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  	return false;
>  }
>  
> -bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> -{
> -	kvm_pfn_t pfn = pte_pfn(range->arg.pte);
> -
> -	if (!kvm->arch.mmu.pgt)
> -		return false;
> -
> -	WARN_ON(range->end - range->start != 1);
> -
> -	/*
> -	 * If the page isn't tagged, defer to user_mem_abort() for sanitising
> -	 * the MTE tags. The S2 pte should have been unmapped by
> -	 * mmu_notifier_invalidate_range_end().
> -	 */
> -	if (kvm_has_mte(kvm) && !page_mte_tagged(pfn_to_page(pfn)))
> -		return false;
> -
> -	/*
> -	 * We've moved a page around, probably through CoW, so let's treat
> -	 * it just like a translation fault and the map handler will clean
> -	 * the cache to the PoC.
> -	 *
> -	 * The MMU notifiers will have unmapped a huge PMD before calling
> -	 * ->change_pte() (which in turn calls kvm_set_spte_gfn()) and
> -	 * therefore we never need to clear out a huge PMD through this
> -	 * calling path and a memcache is not required.
> -	 */
> -	kvm_pgtable_stage2_map(kvm->arch.mmu.pgt, range->start << PAGE_SHIFT,
> -			       PAGE_SIZE, __pfn_to_phys(pfn),
> -			       KVM_PGTABLE_PROT_R, NULL, 0);
> -
> -	return false;
> -}
> -
>  bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	u64 size = (range->end - range->start) << PAGE_SHIFT;

Thanks. It's nice to see this code retire:

Acked-by: Will Deacon <will@...nel.org>

Also, if you're in the business of hacking the MMU notifier code, it
would be really great to change the .clear_flush_young() callback so
that the architecture could handle the TLB invalidation. At the moment,
the core KVM code invalidates the whole VMID courtesy of 'flush_on_ret'
being set by kvm_handle_hva_range(), whereas we could do a much
lighter-weight and targetted TLBI in the architecture page-table code
when we actually update the ptes for small ranges.

Will