linux-kernel - Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka NPT-Virtualization)

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Message-ID: <4B9DE2FF.4080909@redhat.com>
Date:	Mon, 15 Mar 2010 09:34:23 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Joerg Roedel <joerg.roedel@....com>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/18][RFC] Nested Paging support for Nested SVM (aka NPT-Virtualization)

On 03/15/2010 08:27 AM, Marcelo Tosatti wrote:
>>
>>> You should disable out of sync shadow so that l1 guest writes to
>>> l1-nested-ptables always trap.
>>>        
>> Why?  The guest is under obligation to flush the tlb if it writes to
>> a page table, and we will resync on that tlb flush.
>>      
> The guests hypervisor will not flush the tlb with invlpg for updates of
> its NPT pagetables. It'll create a new ASID, and KVM will not trap
> that.
>    

We'll get a kvm_set_cr3() on the next vmrun.

>>> And in the trap case, you'd have to
>>> invalidate l2 shadow pagetable entries that used the (now obsolete)
>>> l1-nested-ptable entry. Does that happen automatically?
>>>        
>> What do you mean by 'l2 shadow ptable entries'?  There are the
>> guest's page tables (ordinary direct mapped, unless the guest's
>> guest is also running an npt-enabled hypervisor), and the host page
>> tables.  When the guest writes to each page table, we invalidate the
>> shadows.
>>      
> With 'l2 shadow ptable entries' i mean the shadow pagetables that
> translate GPA-L2 ->  HPA.
>    

kvm_mmu_pte_write() will invalidate those sptes and will also install 
new translations if possible.

Beautiful, isn't it?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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