linux-kernel - Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

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Date: Wed, 15 May 2024 23:21:57 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>
CC: "sagis@...gle.com" <sagis@...gle.com>, "dmatlack@...gle.com"
	<dmatlack@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>
Subject: Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for
 TDX shared bit of GPA

On Thu, 2024-05-16 at 10:34 +1200, Huang, Kai wrote:
> 
> 
> On 15/05/2024 12:59 pm, Rick Edgecombe wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Introduce a "gfn_shared_mask" field in the kvm_arch structure to record GPA
> > shared bit and provide address conversion helpers for TDX shared bit of
> > GPA.
> > 
> > TDX designates a specific GPA bit as the shared bit, which can be either
> > bit 51 or bit 47 based on configuration.
> > 
> > This GPA shared bit indicates whether the corresponding physical page is
> > shared (if shared bit set) or private (if shared bit cleared).
> > 
> > - GPAs with shared bit set will be mapped by VMM into conventional EPT,
> >     which is pointed by shared EPTP in TDVMCS, resides in host VMM memory
> >     and is managed by VMM.
> > - GPAs with shared bit cleared will be mapped by VMM firstly into a
> >     mirrored EPT, which resides in host VMM memory. Changes of the mirrored
> >     EPT are then propagated into a private EPT, which resides outside of
> > host
> >     VMM memory and is managed by TDX module.
> > 
> > Add the "gfn_shared_mask" field to the kvm_arch structure for each VM with
> > a default value of 0. It will be set to the position of the GPA shared bit
> > in GFN through TD specific initialization code.
> > 
> > Provide helpers to utilize the gfn_shared_mask to determine whether a GPA
> > is shared or private, retrieve the GPA shared bit value, and insert/strip
> > shared bit to/from a GPA.
> 
> I am seriously thinking whether we should just abandon this whole 
> kvm_gfn_shared_mask() thing.
> 
> We already have enough mechanisms around private memory and the mapping 
> of it:
> 
> 1) Xarray to query whether a given GFN is private or shared;
> 2) fault->is_private to indicate whether a faulting address is private 
> or shared;
> 3) sp->is_private to indicate whether a "page table" is only for private 
> mapping;

You mean drop the helpers, or the struct kvm member? I think we still need the
shared bit position stored somewhere. memslots, Xarray, etc need to operate on
the GFN without the shared it.