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 16:27:45 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "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>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "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 Fri, 2024-05-24 at 00:55 -0700, Isaku Yamahata wrote:
> 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 non-TDX VM's won't need per-VM data, right? So it's just unneeded extra
state per-vm.

> 
> 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.

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.