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From: Sean Christopherson <seanjc@google.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Alistair Popple <apopple@nvidia.com>,
	Anup Patel <anup@brainfault.org>, Ben Gardon <bgardon@google.com>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Chao Peng <chao.p.peng@linux.intel.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Fabiano Rosas <farosas@linux.ibm.com>,
	Gaosheng Cui <cuigaosheng1@huawei.com>,
	Gavin Shan <gshan@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Ingo Molnar <mingo@redhat.com>, James Morse <james.morse@arm.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Jonathan Corbet <corbet@lwn.net>,
	Marc Zyngier <maz@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Michael Larabel <michael@michaellarabel.com>,
	Mike Rapoport <rppt@kernel.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Paul Mackerras <paulus@ozlabs.org>, Peter Xu <peterx@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Thomas Huth <thuth@redhat.com>, Will Deacon <will@kernel.org>,
	Zenghui Yu <yuzenghui@huawei.com>,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linuxppc-dev@lists.ozlabs.org,
	linux-trace-kernel@vger.kernel.org, x86@kernel.org,
	linux-mm@google.com
Subject: Re: [PATCH mm-unstable v2 09/10] kvm/x86: add kvm_arch_test_clear_young()
Date: Thu, 15 Jun 2023 11:26:51 -0700	[thread overview]
Message-ID: <ZItX64Bbx5vdjo9M@google.com> (raw)
In-Reply-To: <20230526234435.662652-10-yuzhao@google.com>

On Fri, May 26, 2023, Yu Zhao wrote:
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 08340219c35a..6875a819e007 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1232,6 +1232,40 @@ bool kvm_tdp_mmu_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  	return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn);
>  }
>  
> +bool kvm_arch_test_clear_young(struct kvm *kvm, struct kvm_gfn_range *range)
> +{
> +	struct kvm_mmu_page *root;
> +	int offset = ffs(shadow_accessed_mask) - 1;
> +
> +	if (kvm_shadow_root_allocated(kvm))

This needs a comment.

> +		return true;
> +
> +	rcu_read_lock();
> +
> +	list_for_each_entry_rcu(root, &kvm->arch.tdp_mmu_roots, link) {

As requested in v1[1], please add a macro for a lockless walk.

[1] https://lkml.kernel.org/r/Y%2Fed0XYAPx%2B7pukA%40google.com

> +		struct tdp_iter iter;
> +
> +		if (kvm_mmu_page_as_id(root) != range->slot->as_id)
> +			continue;
> +
> +		tdp_root_for_each_leaf_pte(iter, root, range->start, range->end) {
> +			u64 *sptep = rcu_dereference(iter.sptep);
> +
> +			VM_WARN_ON_ONCE(!page_count(virt_to_page(sptep)));

Hrm, I don't like adding this in KVM.  The primary MMU might guarantee that this
callback is invoked if and only if the SPTE is backed by struct page memory, but
there's no reason to assume that's true in KVM.  If we want the sanity check, then
this needs to use kvm_pfn_to_refcounted_page().

And it should use KVM's MMU_WARN_ON(), which is a mess and effectively dead code,
but I'm working on changing that[*], i.e. by the time this gets to Linus' tree,
the sanity check should have a much cleaner implementation.

[2] https://lore.kernel.org/all/20230511235917.639770-8-seanjc@google.com

> +
> +			if (!(iter.old_spte & shadow_accessed_mask))
> +				continue;
> +
> +			if (kvm_should_clear_young(range, iter.gfn))
> +				clear_bit(offset, (unsigned long *)sptep);

If/when you rebase on https://github.com/kvm-x86/linux/tree/next, can you pull
out the atomic bits of tdp_mmu_clear_spte_bits() and use that new helper? E.g.

diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index fae559559a80..914c34518829 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -58,15 +58,18 @@ static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
        return old_spte;
 }
 
+static inline u64 tdp_mmu_clear_spte_bits_atomic(tdp_ptep_t sptep, u64 mask)
+{
+       atomic64_t *sptep_atomic = (atomic64_t *)rcu_dereference(sptep);
+
+       return (u64)atomic64_fetch_and(~mask, sptep_atomic);
+}
+
 static inline u64 tdp_mmu_clear_spte_bits(tdp_ptep_t sptep, u64 old_spte,
                                          u64 mask, int level)
 {
-       atomic64_t *sptep_atomic;
-
-       if (kvm_tdp_mmu_spte_need_atomic_write(old_spte, level)) {
-               sptep_atomic = (atomic64_t *)rcu_dereference(sptep);
-               return (u64)atomic64_fetch_and(~mask, sptep_atomic);
-       }
+       if (kvm_tdp_mmu_spte_need_atomic_write(old_spte, level))
+               return tdp_mmu_clear_spte_bits_atomic(sptep, mask);
 
        __kvm_tdp_mmu_write_spte(sptep, old_spte & ~mask);
        return old_spte;


  parent reply	other threads:[~2023-06-15 18:26 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-26 23:44 [PATCH mm-unstable v2 00/10] mm/kvm: locklessly clear the accessed bit Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 01/10] mm/kvm: add mmu_notifier_ops->test_clear_young() Yu Zhao
2023-06-06  8:34   ` Tzung-Bi Shih
2023-06-09  1:00     ` Yu Zhao
     [not found]   ` <ZHedMX470b7EMwbe@ziepe.ca>
2023-06-09  9:04     ` Paolo Bonzini
2023-06-15 17:42   ` Sean Christopherson
2023-06-20  7:30   ` Nicholas Piggin
2023-05-26 23:44 ` [PATCH mm-unstable v2 02/10] mm/kvm: use mmu_notifier_ops->test_clear_young() Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 03/10] kvm/arm64: export stage2_try_set_pte() and macros Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe Yu Zhao
2023-05-27 18:08   ` Oliver Upton
2023-05-27 20:13     ` Yu Zhao
2023-05-30 19:37       ` Oliver Upton
2023-05-30 20:06         ` Yu Zhao
     [not found]           ` <ZHef0VsZvZ1Vnz0u@linux.dev>
2023-05-31 23:10             ` Yu Zhao
2023-05-31 23:22               ` Oliver Upton
2023-05-31 23:41                 ` Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 05/10] kvm/arm64: add kvm_arch_test_clear_young() Yu Zhao
2023-05-26 23:44 ` [PATCH mm-unstable v2 06/10] kvm/powerpc: make radix page tables RCU safe Yu Zhao
2023-06-20  6:32   ` Nicholas Piggin
2023-06-20  8:00     ` Yu Zhao
2023-06-20 10:49       ` Nicholas Piggin
2023-05-26 23:44 ` [PATCH mm-unstable v2 07/10] kvm/powerpc: add kvm_arch_test_clear_young() Yu Zhao
2023-06-20  7:47   ` Nicholas Piggin
2023-06-21  0:38     ` Yu Zhao
2023-06-21  2:51       ` Nicholas Piggin
2023-05-26 23:44 ` [PATCH mm-unstable v2 08/10] kvm/x86: move tdp_mmu_enabled and shadow_accessed_mask Yu Zhao
2023-06-15 16:59   ` Sean Christopherson
2023-05-26 23:44 ` [PATCH mm-unstable v2 09/10] kvm/x86: add kvm_arch_test_clear_young() Yu Zhao
2023-06-09  9:06   ` Paolo Bonzini
2023-06-15 18:26   ` Sean Christopherson [this message]
2023-05-26 23:44 ` [PATCH mm-unstable v2 10/10] mm: multi-gen LRU: use mmu_notifier_test_clear_young() Yu Zhao
2023-06-09  0:59 ` kvm/arm64: Spark benchmark Yu Zhao
2023-06-09 13:04   ` Marc Zyngier
2023-06-18 20:11     ` Yu Zhao
2023-06-09  0:59 ` kvm/powerpc: memcached benchmark Yu Zhao
2023-06-09  0:59 ` kvm/x86: multichase benchmark Yu Zhao
2023-06-18 19:19   ` Yu Zhao
2023-06-09  9:07 ` [PATCH mm-unstable v2 00/10] mm/kvm: locklessly clear the accessed bit Paolo Bonzini
2023-06-20  2:19   ` Yu Zhao

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