linux-kernel - Re: [PATCH 1/2] KVM: x86: fix usage of kvm_lock in set_nx_huge

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Message-ID: <CABgObfY6C=2LnKQSPon7Mi8bFnKhpT87OngjyGLf73s6yeh5Zg@mail.gmail.com>
Date: Mon, 27 Jan 2025 18:27:54 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: x86: fix usage of kvm_lock in set_nx_huge_pages()

On Sat, Jan 25, 2025 at 1:44 AM Sean Christopherson <seanjc@...gle.com> wrote:
> I like the special casing, it makes the oddballs stand out, which in turn (hopefully)
> makes developers pause and take note.  I.e. the SRCU walkers are all normal readers,
> the set_nx_huge_pages() "never" path is a write in disguise, and
> kvm_hyperv_tsc_notifier() is a very special snowflake.

set_nx_huge_pages() is not a writer in disguise.  Rather, it's
a *real* writer for nx_hugepage_mitigation_hard_disabled which is
also protected by kvm_lock; and there's a (mostly theoretical)
bug in set_nx_huge_pages_recovery_param() which reads it without
taking the lock.  But it's still a reader as far as vm_list is
concerned.

Likewise, kvm_hyperv_tsc_notifier()'s requirement does deserve a comment,
but its specialness is self-inflicted pain due to using (S)RCU even when
it's not the most appropriate synchronization mechanism.

Paolo