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

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Date:   Tue, 10 Aug 2021 19:00:31 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, 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 Tue, Aug 10, 2021 at 11:25:27AM +0200, Paolo Bonzini wrote:
> On 10/08/21 09:40, Yu Zhang wrote:
> > About "host can't easily mirror L1's desired paging mode", could you please elaborate?
> > Thanks!
> 
> Shadow pgae tables in KVM will always have 3 levels on 32-bit machines and
> 4/5 levels on 64-bit machines.  L1 instead might have any number of levels
> from 2 to 5 (though of course not more than the host has).

Thanks Paolo.

I guess it's because, unlike EPT which are with either 4 or 5 levels, NPT's
level can range from 2 to 5, depending on the host paging mode...

> 
> Therefore, when shadowing 32-bit NPT page tables, KVM has to add extra fixed
> levels on top of those that it's shadowing.  See mmu_alloc_direct_roots for
> the code.

So when shadowing NPTs(can be 2/3 levels, depending on the paging mode in L1),
and if L0 Linux is running in 4/5 level mode, extra levels of paging structures
is needed in the shadow NPT.

But shadow EPT does not have such annoyance. Is above understanding correct?

B.R.
Yu