linux-kernel - Re: [PATCH mm-unstable v1 1/5] mm/kvm: add mmu_notifier_test_clear

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Date:   Thu, 23 Feb 2023 09:34:52 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Michael Larabel <michael@...haellarabel.com>,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-mm@...gle.com
Subject: Re: [PATCH mm-unstable v1 1/5] mm/kvm: add mmu_notifier_test_clear_young()

On Thu, Feb 16, 2023, Yu Zhao wrote:
> +static bool kvm_mmu_notifier_test_clear_young(struct mmu_notifier *mn, struct mm_struct *mm,
> +					      unsigned long start, unsigned long end,
> +					      unsigned long *bitmap)
> +{
> +	if (kvm_arch_has_test_clear_young())
> +		return kvm_test_clear_young(mmu_notifier_to_kvm(mn), start, end, bitmap);
> +
> +	return false;
> +}
> +
>  static int kvm_mmu_notifier_test_young(struct mmu_notifier *mn,
>  				       struct mm_struct *mm,
>  				       unsigned long address)
> @@ -903,6 +960,7 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
>  	.clear_flush_young	= kvm_mmu_notifier_clear_flush_young,
>  	.clear_young		= kvm_mmu_notifier_clear_young,
>  	.test_young		= kvm_mmu_notifier_test_young,
> +	.test_clear_young	= kvm_mmu_notifier_test_clear_young,

I am strongly opposed to adding yet another "young" mmu_notifier hook for this.
I would much rather we extend (and rename) mmu_notifier_clear_young() to take the
bitmap as an optional parameter, and then have KVM hide the details of whether or
not it supports lockless processing of the range/bitmap.

I also think for KVM x86 in particular, this series should first convert to a
lockless walk for aging ranges, and then add the batching.  It might also make
sense to land x86 first and then follow-up with ARM and PPC.  I haven't looked at
the ARM or PPC patches in too much depth, but on the x86 side of things KVM already
has the pieces in place to support a fully lockless walk, i.e. the x86 specific
changes aren't all that contentious, the only thing we need to figure out is what
to do about nested VMs.