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

Paolo Bonzini
2022-03-02T18:20:22Z

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