linux-kernel - Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP level

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Message-ID: <20210809064224.ctu3zxknn7s56gk3@linux.intel.com>
Date:   Mon, 9 Aug 2021 14:42:24 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Wei Huang <wei.huang2@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com
Subject: Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP
 level

On Sun, Aug 08, 2021 at 11:33:44PM -0500, Wei Huang wrote:
> 
> 
> On 8/8/21 11:27 PM, Yu Zhang wrote:
> > On Sun, Aug 08, 2021 at 11:11:40PM -0500, Wei Huang wrote:
> > > 
> > > 
> > > On 8/8/21 10:58 PM, Yu Zhang wrote:
> > > > On Sun, Aug 08, 2021 at 02:26:56PM -0500, Wei Huang wrote:
> > > > > AMD future CPUs will require a 5-level NPT if host CR4.LA57 is set.
> > > > 
> > > > Sorry, but why? NPT is not indexed by HVA.
> > > 
> > > NPT is not indexed by HVA - it is always indexed by GPA. What I meant is NPT
> > > page table level has to be the same as the host OS page table: if 5-level
> > > page table is enabled in host OS (CR4.LA57=1), guest NPT has to 5-level too.
> > 
> > I know what you meant. But may I ask why?
> 
> I don't have a good answer for it. From what I know, VMCB doesn't have a
> field to indicate guest page table level. As a result, hardware relies on
> host CR4 to infer NPT level.

I guess you mean not even in the N_CR3 field of VMCB? 

Then it's not a broken design - it's a limitation of SVM. :)

B.R.
Yu