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

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Date: Fri, 24 May 2024 00:55:19 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"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>,
	"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>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Thu, May 23, 2024 at 06:27:49PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Wed, 2024-05-22 at 17:01 -0700, Isaku Yamahata wrote:
> > Ok, Let's include the patch.
> 
> We were discussing offline, that actually the existing behavior of
> kvm_mmu_max_gfn() can be improved for normal VMs. It would be more proper to
> trigger it off of the GFN range supported by EPT level, than the host MAXPA. 
> 
> Today I was thinking, to fix this would need somthing like an x86_ops.max_gfn(),
> so it could get at VMX stuff (usage of 4/5 level EPT). If that exists we might
> as well just call it directly in kvm_mmu_max_gfn().
> 
> Then for TDX we could just provide a TDX implementation, rather than stash the
> GFN on the kvm struct? Instead it could use gpaw stashed on struct kvm_tdx. The
> op would still need to be take a struct kvm.
> 
> What do you think of that alternative?

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 max_gfn for the guest is rather static once the guest is created and
initialized.  Also the existing codes that use max_gfn expect that the value
doesn't change.  So we can use x86_ops.vm_init() to determine the value for VMX
and TDX.  If we introduced x86_ops.max_gfn(), the implementation will be simply
return kvm_vmx->max_gfn or return kvm_tdx->max_gfn. (We would have similar for
SVM and SEV.)  So I don't see benefit of x86_ops.max_gfn() than
kvm->arch.max_gfn.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>