linux-kernel - Re: [PATCH v4 4/7] KVM: Move MMU lock acquisition for test/clear

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Date: Wed, 29 May 2024 14:55:46 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Ankit Agrawal <ankita@...dia.com>, 
	Anup Patel <anup@...infault.org>, Atish Patra <atishp@...shpatra.org>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, Bibo Mao <maobibo@...ngson.cn>, 
	Catalin Marinas <catalin.marinas@....com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Huacai Chen <chenhuacai@...nel.org>, 
	James Morse <james.morse@....com>, Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>, 
	Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Palmer Dabbelt <palmer@...belt.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Raghavendra Rao Ananta <rananta@...gle.com>, 
	Ryan Roberts <ryan.roberts@....com>, Shaoqin Huang <shahuang@...hat.com>, 
	Shuah Khan <shuah@...nel.org>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>, 
	Zenghui Yu <yuzenghui@...wei.com>, kvm-riscv@...ts.infradead.org, kvm@...r.kernel.org, 
	kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mips@...r.kernel.org, 
	linux-mm@...ck.org, linux-riscv@...ts.infradead.org, 
	linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev
Subject: Re: [PATCH v4 4/7] KVM: Move MMU lock acquisition for
 test/clear_young to architecture

On Wed, May 29, 2024, James Houghton wrote:
> For implementation mmu_notifier_{test,clear}_young, the KVM memslot
> walker used to take the MMU lock for us. Now make the architectures
> take it themselves.

Hmm, *forcing* architectures to take mmu_lock is a step backwards.  Rather than
add all of this churn, what about adding CONFIG_KVM_MMU_NOTIFIER_LOCKLESS, e.g.

static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn,
							 unsigned long start,
							 unsigned long end,
							 gfn_handler_t handler)
{
	struct kvm *kvm = mmu_notifier_to_kvm(mn);
	const struct kvm_mmu_notifier_range range = {
		.start		= start,
		.end		= end,
		.handler	= handler,
		.on_lock	= (void *)kvm_null_fn,
		.flush_on_ret	= false,
		.may_block	= false,
		.lockless	= IS_ENABLED(CONFIG_KVM_MMU_NOTIFIER_LOCKLESS),
	};

	return __kvm_handle_hva_range(kvm, &range).ret;
}