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

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Message-ID: <20200902151848.GA11695@sjchrist-ice>
Date:   Wed, 2 Sep 2020 08:18:48 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.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 Tue, Sep 01, 2020 at 12:36:40PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@...el.com> writes:
> 
> > 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.
> 
> But we can't hit this BUG_ON(), right? I'm failing to see how
> hardware_disable_all_nolock() can be reached with kvm_usage_count==0.

Correct, I was mostly talking to myself.