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

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Date: Wed, 29 May 2024 02:13:52 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>
CC: "isaku.yamahata@...il.com" <isaku.yamahata@...il.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>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Tue, 2024-05-28 at 19:47 +0200, Paolo Bonzini wrote:
> 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.

Ok. Since this has (optimization) utility beyond TDX, maybe it's worth splitting
it off as a separate patch? I think maybe we'll pursue this path unless there is
objection.

> 
> > 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.

I meant for a TDX specific x86_ops implementation we already have the data
needed to compute it (gfn_shared_mask - 1). I didn't realize SEV would benefit
from this too.