linux-kernel - Re: [PATCH 02/18] KVM: MMU: Make tdp

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Message-ID: <4B9891F9.6080706@redhat.com>
Date:	Thu, 11 Mar 2010 08:47:21 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joerg.roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Alexander Graf <agraf@...e.de>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/18] KVM: MMU: Make tdp_enabled a mmu-context parameter

On 03/10/2010 05:26 PM, Joerg Roedel wrote:
> On Wed, Mar 10, 2010 at 04:53:29PM +0200, Avi Kivity wrote:
>    
>> On 03/10/2010 04:44 PM, Joerg Roedel wrote:
>>      
>>> On Mon, Mar 08, 2010 at 11:17:41AM +0200, Avi Kivity wrote:
>>>        
>>>> On 03/03/2010 09:12 PM, Joerg Roedel wrote:
>>>>          
>>>>> This patch changes the tdp_enabled flag from its global
>>>>> meaning to the mmu-context. This is necessary for Nested SVM
>>>>> with emulation of Nested Paging where we need an extra MMU
>>>>> context to shadow the Nested Nested Page Table.
>>>>>
>>>>>
>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>>> index ec891a2..e7bef19 100644
>>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>>> @@ -254,6 +254,7 @@ struct kvm_mmu {
>>>>>   	int root_level;
>>>>>   	int shadow_root_level;
>>>>>   	union kvm_mmu_page_role base_role;
>>>>> +	bool tdp_enabled;
>>>>>
>>>>>            
>>>> This needs a different name, since the old one is still around.
>>>> Perhaps we could call it parent_mmu and make it a kvm_mmu pointer.
>>>>          
>>> Hmm, how about renaming the global tdp_enabled variable to tdp_usable?
>>> The global variable indicates if tdp is _usable_ and we can _enable_ it
>>> for a mmu context.
>>>        
>> I think of the global flags as host tdp, and the mmu as guest tdp
>> (but maybe this is wrong?).  If that makes sense, the naming should
>> reflect that.
>>      
> The basic flow of the mmu state with npt-npt is:
>
> 	1. As long as the L1 is running the arch.mmu context is in tdp
> 	   mode and builds a direct-mapped page table.
> 	
> 	2. When vmrun is emulated and the nested vmcb enables nested
> 	   paging, arch.mmu is switched to a shadow-mmu mode which now
> 	   shadows the l1 nested page table.
> 	   So when the l2-guest runs with nested paging the
> 	   arch.mmu.tdp_enabled variable on the host is false.
> 	
> 	3. On a vmexit emulation the mmu is switched back to tdp
> 	   handling state.
>
> So the mmu.tdp_enabled parameter is about tdp being enabled for the
> mmu context (so mmu.tdp_enabled means that we build a l1-direct-mapped
> page table when true or shadow a l1-page-table when false). Thats why I
> think the 'tdp_enabled' name makes sense in the mmu-context.
> The global flag only shows if an mmu-context could be in tdp-state. So
> tdp_usable may be a good name for it.
>
>    

tdp is still used in both cases, so that name is confusing.  We could 
call it mmu.direct_map (and set it for real mode?) or mmu.virtual_map 
(with the opposite sense).  Or something.

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

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