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

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Message-ID: <20100310152643.GO5619@amd.com>
Date:	Wed, 10 Mar 2010 16:26:43 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.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 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.

	Joerg


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