linux-kernel - 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

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Message-ID: <56E16527.4020908@linux.intel.com>
Date:	Thu, 10 Mar 2016 20:14:31 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	stable@...r.kernel.org, Andy Lutomirski <luto@...capital.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



On 03/10/2016 06:09 PM, Paolo Bonzini wrote:
>
>
> On 10/03/2016 09:27, Xiao Guangrong wrote:
>>>
>>
>>> +    if (!enable_ept) {
>>> +        guest_efer |= EFER_NX;
>>> +        ignore_bits |= EFER_NX;
>>
>> Update ignore_bits is not necessary i think.
>
> 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.

>
> This is what I came up with (plus some comments :)):
>
> 	u64 guest_efer = vmx->vcpu.arch.efer;
> 	u64 ignore_bits = 0;
>
> 	if (!enable_ept) {
> 		if (boot_cpu_has(X86_FEATURE_SMEP))
> 			guest_efer |= EFER_NX;
> 		else if (!(guest_efer & EFER_NX))
> 			ignore_bits |= EFER_NX;
> 	}

Your logic is very right.

What my suggestion is we can keep ignore_bits = EFER_NX | EFER_SCE;
(needn't conditionally adjust it) because EFER_NX must be the same
between guest and host if we switch EFER manually.

> My patch is bigger but the resulting code is smaller and easier to follow:
>
> 	guest_efer = vmx->vcpu.arch.efer;
> 	if (!enable_ept)
> 		guest_efer |= EFER_NX;
> 	...
> 	if (...) {
> 		...
> 	} else {
> 		guest_efer &= ~ignore_bits;
> 		guest_efer |= host_efer & ignore_bits;
> 	}

I agreed. :)