linux-kernel - Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous worker

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Date:   Wed, 2 Mar 2022 19:20:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 22/28] KVM: x86/mmu: Zap defunct roots via asynchronous
 worker

On 3/2/22 19:01, Sean Christopherson wrote:
>> +	 */
>> +	if (!refcount_read(&kvm->users_count)) {
>> +		kvm_mmu_zap_all(kvm);
>> +		return;
>> +	}
> 
> I'd prefer we make this an assertion and shove this logic to set_nx_huge_pages(),
> because in that case there's no need to zap anything, the guest can never run
> again.  E.g. (I'm trying to remember why I didn't do this before...)

I did it this way because it seemed like a reasonable fallback for any 
present or future caller.

> One thing that keeps tripping me up is the "readers" verbiage.  I get confused
> because taking mmu_lock for read vs. write doesn't really have anything to do with
> reading or writing state, e.g. "readers" still write SPTEs, and so I keep thinking
> "readers" means anything iterating over the set of roots.  Not sure if there's a
> shorthand that won't be confusing.

Not that I know of.  You really need to know that the rwlock is been 
used for its shared/exclusive locking behavior.  But even on ther OSes 
use shared/exclusive instead of read/write, there are no analogous nouns 
and people end up using readers/writers anyway.

>> It passes a smoke test, and also resolves the debate on the fate of patch 1.
> +1000, I love this approach.  Do you want me to work on a v3, or shall I let you
> have the honors?

I'm already running the usual battery of tests, so I should be able to 
post it either tomorrow (early in my evening) or Friday morning.

Paolo