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

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Date: Fri, 19 Apr 2024 13:54:47 -0700
From: James Houghton <jthoughton@...gle.com>
To: David Matlack <dmatlack@...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 Fri, Apr 12, 2024 at 1:44 PM David Matlack <dmatlack@...gle.com> wrote:
>
> 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.

Indeed, thanks. I'll make sure to build with CONFIG_LOCKDEP for the
future versions and check for errors.

>
> 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.

I'll add the macro / function to do the lockless walk of tdp_mmu_roots
and explain why it is safe. Thanks for pointing out this big mistake.

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