Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging

From: David Matlack <dmatlack@google.com>
To: James Houghton <jthoughton@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Paolo Bonzini <pbonzini@redhat.com>, Yu Zhao <yuzhao@google.com>,
	Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	Sean Christopherson <seanjc@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Shaoqin Huang <shahuang@redhat.com>,
	Gavin Shan <gshan@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	Ryan Roberts <ryan.roberts@arm.com>,
	David Rientjes <rientjes@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, linux-mm@kvack.org,
	linux-trace-kernel@vger.kernel.org
Subject: Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging
Date: Fri, 12 Apr 2024 13:44:41 -0700	[thread overview]
Message-ID: <ZhmdOb7lAd3DM5Tq@google.com> (raw)
In-Reply-To: <20240401232946.1837665-6-jthoughton@google.com>

On 2024-04-01 11:29 PM, James Houghton wrote:
> Only handle the TDP MMU case for now. In other cases, if a bitmap was
> not provided, fallback to the slowpath that takes mmu_lock, or, if a
> bitmap was provided, inform the caller that the bitmap is unreliable.

I think this patch will trigger a lockdep assert in

  kvm_tdp_mmu_age_gfn_range
    kvm_tdp_mmu_handle_gfn
      for_each_tdp_mmu_root
        __for_each_tdp_mmu_root
          kvm_lockdep_assert_mmu_lock_held

... because it walks tdp_mmu_roots without holding mmu_lock.

Yu's patch[1] added a lockless walk to the TDP MMU. We'd need something
similar here and also update the comment above tdp_mmu_roots describing
how tdp_mmu_roots can be read locklessly.

[1] https://lore.kernel.org/kvmarm/ZItX64Bbx5vdjo9M@google.com/