Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Lai Jiangshan <jiangshan.ljs@antgroup.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-doc@vger.kernel.org
Subject: Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable
Date: Wed, 13 Apr 2022 10:38:26 +0200	[thread overview]
Message-ID: <caffa434-5644-ee73-1636-45a87517bae2@redhat.com> (raw)
In-Reply-To: <YlXvtMqWpyM9Bjox@google.com>

On 4/12/22 23:31, Sean Christopherson wrote:
>> +		unsigned glevel:4;
> We don't need 4 bits for this.  Crossing our fingers that we never had to shadow
> a 2-level guest with a 6-level host, we can do:
> 
> 		unsigned passthrough_delta:2;
> 
> Where the field is ignored if direct=1, '0' for non-passthrough, and 1-3 to handle
> shadow_root_level - guest_root_level.  Basically the same idea as Paolo's smushing
> of direct+passthrough into mapping_level, just dressed up differently.

Basically, your passthrough_delta is level - glevel in Jiangshan's 
patches.  You'll need 3 bits anyway when we remove direct later (that 
would be passthrough_delta == level).

Regarding the naming:

* If we keep Jiangshan's logic, I don't like the glevel name very much, 
any of mapping_level, target_level or direct_level would be clearer?

* If we go with yours, I would call the field "passthrough_levels".

Paolo