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

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Date: Fri, 12 Apr 2024 13:44:41 -0700
From: David Matlack <dmatlack@...gle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Paolo Bonzini <pbonzini@...hat.com>, Yu Zhao <yuzhao@...gle.com>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Sean Christopherson <seanjc@...gle.com>,
	Jonathan Corbet <corbet@....net>, James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Shaoqin Huang <shahuang@...hat.com>, Gavin Shan <gshan@...hat.com>,
	Ricardo Koller <ricarkol@...gle.com>,
	Raghavendra Rao Ananta <rananta@...gle.com>,
	Ryan Roberts <ryan.roberts@....com>,
	David Rientjes <rientjes@...gle.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging

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/