linux-kernel - Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

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Date:   Mon, 24 Aug 2020 17:09:20 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

On Mon, Aug 24, 2020 at 03:45:26PM -0700, Jim Mattson wrote:
> On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@...gle.com> wrote:
> >
> > On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > > I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
> > > pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
> > > impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
> > > finished.
> >
> > Unless I'm missing something, there's no check for a non-zero
> > kvm_usage_count on this path. There is such a check in
> > hardware_disable_all_nolock(), but not in hardware_disable_nolock().
> 
> However, cpus_hardware_enabled shouldn't have any bits set, so
> everything's fine. Nothing to see here, after all.

Ugh, I forgot that hardware_disable_all_nolock() does a BUG_ON() instead of
bailing on !kvm_usage_count.