Re: [PATCH v5 3/9] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Jun 11, 2024 at 12:21:39AM +0000, James Houghton wrote:
> Replace the MMU write locks (taken in the memslot iteration loop) for
> read locks.
> 
> Grabbing the read lock instead of the write lock is safe because the
> only requirement we have is that the stage-2 page tables do not get
> deallocated while we are walking them. The stage2_age_walker() callback
> is safe to race with itself; update the comment to reflect the
> synchronization change.
> 
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>  arch/arm64/kvm/Kconfig       |  1 +
>  arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++------
>  arch/arm64/kvm/mmu.c         | 26 ++++++++++++++++++++------
>  3 files changed, 30 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 58f09370d17e..7a1af8141c0e 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -22,6 +22,7 @@ menuconfig KVM
>  	select KVM_COMMON
>  	select KVM_GENERIC_HARDWARE_ENABLING
>  	select KVM_GENERIC_MMU_NOTIFIER
> +	select KVM_MMU_NOTIFIER_YOUNG_LOCKLESS
>  	select HAVE_KVM_CPU_RELAX_INTERCEPT
>  	select KVM_MMIO
>  	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 9e2bbee77491..b1b0f7148cff 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -1319,10 +1319,10 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
>  	data->young = true;
>  
>  	/*
> -	 * stage2_age_walker() is always called while holding the MMU lock for
> -	 * write, so this will always succeed. Nonetheless, this deliberately
> -	 * follows the race detection pattern of the other stage-2 walkers in
> -	 * case the locking mechanics of the MMU notifiers is ever changed.
> +	 * This walk may not be exclusive; the PTE is permitted to change

s/may not/is not/

> +	 * from under us. If there is a race to update this PTE, then the
> +	 * GFN is most likely young, so failing to clear the AF is likely
> +	 * to be inconsequential.
>  	 */
>  	if (data->mkold && !stage2_try_set_pte(ctx, new))
>  		return -EAGAIN;
> @@ -1345,10 +1345,13 @@ bool kvm_pgtable_stage2_test_clear_young(struct kvm_pgtable *pgt, u64 addr,
>  	struct kvm_pgtable_walker walker = {
>  		.cb		= stage2_age_walker,
>  		.arg		= &data,
> -		.flags		= KVM_PGTABLE_WALK_LEAF,
> +		.flags		= KVM_PGTABLE_WALK_LEAF |
> +				  KVM_PGTABLE_WALK_SHARED,
>  	};
> +	int r;
>  
> -	WARN_ON(kvm_pgtable_walk(pgt, addr, size, &walker));
> +	r = kvm_pgtable_walk(pgt, addr, size, &walker);
> +	WARN_ON(r && r != -EAGAIN);

I could've been more explicit last time around, could you please tone
this down to WARN_ON_ONCE() as well?

>  	return data.young;
>  }
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 8bcab0cc3fe9..a62c27a347ed 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1773,25 +1773,39 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  bool kvm_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	u64 size = (range->end - range->start) << PAGE_SHIFT;
> +	bool young = false;
> +
> +	read_lock(&kvm->mmu_lock);
>  
>  	if (!kvm->arch.mmu.pgt)
>  		return false;

I'm guessing you meant to have 'goto out' here, since this early return
fails to drop the mmu_lock.

-- 
Thanks,
Oliver

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