linux-kernel - Re: [PATCH v5 1/9] KVM: MMU: fix forgot reserved bits check in speculative path

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Date:	Mon, 12 Jul 2010 16:15:00 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v5 1/9] KVM: MMU: fix forgot reserved bits check in speculative
 path

On 07/12/2010 05:37 AM, Xiao Guangrong wrote:
>    
>>> +    if (is_rsvd_bits_set(vcpu, gentry, PT_PAGE_TABLE_LEVEL))
>>> +        gentry = 0;
>>> +
>>>
>>>        
>> That only works if the gpte is for the same mode as the current vcpu mmu
>> mode.  In some cases it is too strict (vcpu in pae mode writing a 32-bit
>> gpte), which is not too bad, in some cases it is too permissive (vcpu in
>> nonpae mode writing a pae gpte).
>>
>>      
> Avi, thanks for your review.
>
> Do you mean that the VM has many different mode vcpu? For example, both
> nonpae vcpu and pae vcpu are running in one VM? I forgot to consider this
> case.
>    

Yes.  This happens while the guest brings up other vcpus, and when using 
nested virtualization.

>> (once upon a time mixed modes were rare, only on OS setup, but with
>> nested virt they happen all the time).
>>      
> I'm afraid it's still has problem, it will cause access corruption:
> 1: if nonpae vcpu write pae gpte, it will miss NX bit
> 2: if pae vcpu write nonpae gpte, it will add NX bit that over gpte's width
>
> How about only update the shadow page which has the same pae set with the written
> vcpu? Just like this:
>
> @@ -3000,6 +3000,10 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>                  while (npte--) {
>                          entry = *spte;
>                          mmu_pte_write_zap_pte(vcpu, sp, spte);
> +
> +                       if (!!is_pae(vcpu) != sp->role.cr4_pae)
> +                               continue;
> +
>    

Not enough, one vcpu can have nx set while the other has it reset, etc.

-- 
error compiling committee.c: too many arguments to function

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