linux-kernel - Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

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Date: Tue, 28 May 2024 19:47:39 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>, 
	"seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>, 
	"sagis@...gle.com" <sagis@...gle.com>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"Aktas, Erdem" <erdemaktas@...gle.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Tue, May 28, 2024 at 6:27 PM Edgecombe, Rick P
<rick.p.edgecombe@...el.com> wrote:
> > I don't see benefit of x86_ops.max_gfn() compared to kvm->arch.max_gfn.
> > But I don't have strong preference. Either way will work.
>
> The non-TDX VM's won't need per-VM data, right? So it's just unneeded extra
> state per-vm.

It's just a cached value like there are many in the MMU. It's easier
for me to read code without the mental overhead of a function call.

> For TDX it will be based on the shared bit, so we actually already have the per-
> vm data we need. So we don't even need both gfn_shared_mask and max_gfn for TDX.

But they are independent, for example AMD placed the encryption bit
highest, then the reduced physical address space bits, then finally
the rest of the gfn. I think it's consistent with the kvm_has_*
approach, to not assume much and just store separate data.

Paolo