linux-kernel - Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

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Date:   Wed, 31 Mar 2021 22:27:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

On Wed, Mar 31, 2021, Ben Gardon wrote:
> Provide a real mechanism for fast invalidation by marking roots as
> invalid so that their reference count will quickly fall to zero
> and they will be torn down.
> 
> One negative side affect of this approach is that a vCPU thread will
> likely drop the last reference to a root and be saddled with the work of
> tearing down an entire paging structure. This issue will be resolved in
> a later commit.
> 
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---

...

> +/*
> + * This function depends on running in the same MMU lock cirical section as
> + * kvm_reload_remote_mmus. Since this is in the same critical section, no new
> + * roots will be created between this function and the MMU reload signals
> + * being sent.

Eww.  That goes beyond just adding a lockdep assertion here.  I know you want to
isolate the TDP MMU as much as possible, but this really feels like it should be
open coded in kvm_mmu_zap_all_fast().  And assuming this lands after as_id is
added to for_each_tdp_mmu_root(), it's probably easier to open code anyways, e.g.
use list_for_each_entry() directly instead of bouncing through an iterator.

> + */
> +void kvm_tdp_mmu_invalidate_roots(struct kvm *kvm)
> +{
> +	struct kvm_mmu_page *root;
> +
> +	for_each_tdp_mmu_root(kvm, root)
> +		root->role.invalid = true;
> +}