linux-kernel - Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

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Message-ID: <20200611000032.GI18790@linux.intel.com>
Date:   Wed, 10 Jun 2020 17:00:32 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "David P. Reed" <dpreed@...pplum.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Allison Randal <allison@...utok.net>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Martin Molnar <martin.molnar.programming@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

On Wed, Jun 10, 2020 at 02:59:19PM -0700, Andy Lutomirski wrote:
> 
> 
> > On Jun 10, 2020, at 11:21 AM, David P. Reed <dpreed@...pplum.com> wrote:
> > 
> > If a panic/reboot occurs when CR4 has VMX enabled, a VMXOFF is
> > done on all CPUS, to allow the INIT IPI to function, since
> > INIT is suppressed when CPUs are in VMX root operation.
> > However, VMXOFF causes an undefined operation fault if the CPU is not
> > in VMX operation, that is, VMXON has not been executed, or VMXOFF
> > has been executed, but VMX is enabled.
> 
> I’m surprised. Wouldn’t this mean that emergency reboots always fail it a VM
> is running?  I would think someone would have noticed before.

The call to cpu_vmxoff() is conditioned on CR4.VMXE==1, which KVM toggles in
tandem with VMXON and VMXOFF.  Out of tree hypervisors presumably do the
same.  That's obviously not atomic though, e.g. VMXOFF will #UD if the
vmxoff_nmi() NMI arrives between CR4.VMXE=1 and VMXON, or between VMXOFF
and CR4.VMXE=0.