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

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Message-ID: <87wo4evaj3.fsf@nanos.tec.linutronix.de>
Date:   Thu, 11 Jun 2020 00:08:48 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     "David P. Reed" <dpreed@...pplum.com>,
        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

Sean Christopherson <sean.j.christopherson@...el.com> writes:
> Gah, I typed too slow :-)

Haha. I had the same thought.

> On Wed, Jun 10, 2020 at 11:34:21PM +0200, Thomas Gleixner wrote:
>> We have exception fixups to avoid exactly that kind of horrible
>> workarounds all over the place.
>> 
>> static inline int cpu_vmxoff_safe(void)
>> {
>>         int err;
>>  
>> 	asm volatile("2: vmxoff; xor %[err],%[err]\n"
>> 		     "1:\n\t"
>> 		     ".section .fixup,\"ax\"\n\t"
>> 		     "3:  mov %[fault],%[err] ; jmp 1b\n\t"
>> 		     ".previous\n\t"
>> 		     _ASM_EXTABLE(2b, 3b)
>> 		     : [err] "=a" (err)
>> 		     : [fault] "i" (-EFAULT)
>> 		     : "memory");
>>         return err;
>> }
>> 
>> static inline void __cpu_emergency_vmxoff(void)
>> {
>>         if (!cpu_vmx_enabled())
>>         	return;
>>         if (!cpu_vmxoff_safe())
>>         	cr4_clear_bits(X86_CR4_VMXE);
>
> This bit is wrong, CR4.VMXE should be cleared even if VMXOFF faults, e.g.
> if this is called in NMI context and the NMI arrived in KVM code between
> VMXOFF and clearing CR4.VMXE.

Oh, right.

> All other VMXOFF faults are mode related, i.e. any fault is guaranteed to
> be due to the !post-VMXON check unless we're magically in RM, VM86, compat
> mode, or at CPL>0.

Your patch is simpler indeed.

Thanks,

        tglx