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From: Paolo Bonzini <pbonzini@redhat.com>
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: stable@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH 1/2] KVM: MMU: fix ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo
Date: Thu, 10 Mar 2016 13:26:06 +0100	[thread overview]
Message-ID: <56E167DE.4090602@redhat.com> (raw)
In-Reply-To: <56E16527.4020908@linux.intel.com>



On 10/03/2016 13:14, Xiao Guangrong wrote:
>> More precisely, ignore_bits is only needed if guest EFER.NX=0 and we're
>> not in this CR0.WP=1/CR4.SMEP=0 situation.  In theory you could have
>> guest EFER.NX=1 and host EFER.NX=0.
> 
> It is not in linux, the kernel always set EFER.NX if CPUID reports it,
> arch/x86/kernel/head_64.S:
> 
> 204         /* Setup EFER (Extended Feature Enable Register) */
> 205         movl    $MSR_EFER, %ecx
> 206         rdmsr
> 207         btsl    $_EFER_SCE, %eax        /* Enable System Call */
> 208         btl     $20,%edi                /* No Execute supported? */
> 209         jnc     1f
> 210         btsl    $_EFER_NX, %eax
> 211         btsq    $_PAGE_BIT_NX,early_pmd_flags(%rip)
> 212 1:      wrmsr                           /* Make changes effective */
> 
> So if guest sees NX in its cpuid then host EFER.NX should be 1.

You're right.  It's just in theory.  But ignoring EFER.NX when it is 1
is technically not correct; since we have to add some special EFER_NX
logic anyway, I preferred to make it pedantically right. :)

Paolo

  reply	other threads:[~2016-03-10 12:26 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-08 11:44 [PATCH 0/2] KVM: MMU: fix ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 Paolo Bonzini
2016-03-08 11:44 ` [PATCH 1/2] KVM: MMU: fix ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo Paolo Bonzini
2016-03-10  8:27   ` Xiao Guangrong
2016-03-10 10:01     ` Paolo Bonzini
2016-03-10 10:09     ` Paolo Bonzini
2016-03-10 12:14       ` Xiao Guangrong
2016-03-10 12:26         ` Paolo Bonzini [this message]
2016-03-10  8:46   ` Xiao Guangrong
2016-03-10 10:03     ` Paolo Bonzini
2016-03-08 11:44 ` [PATCH 2/2] KVM: MMU: fix reserved bit check for pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 Paolo Bonzini
2016-03-10  8:36   ` Xiao Guangrong
2016-03-10 10:02     ` Paolo Bonzini

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