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 11:25:27 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 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).

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.

Paolo