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

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Message-ID: <CABgObfb-Q-fgruhEHutdmD00UYtD7mJBsOYhW5Tf4y-Txhr2jQ@mail.gmail.com>
Date: Mon, 27 Jan 2025 18:56:45 +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 Mon, Jan 27, 2025 at 6:27 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> 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.
>
> 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.

... in fact, you could have a KVM_CREATE_VCPU and KVM_RUN after this
point:

        mutex_lock(&kvm_lock);
        list_for_each_entry(kvm, &vm_list, vm_list)
                kvm_make_mclock_inprogress_request(kvm);

because kvm_lock is not enough to ensure that all vCPUs got the
KVM_REQ_MCLOCK_INPROGRESS memo.  So kvm_hyperv_tsc_notifier()'s
complications go beyond kvm_lock and the choice to use SRCU or not.

Paolo