linux-kernel - Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of make

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Message-ID: <80407e4a-36e1-e606-ed9f-74429f850e77@redhat.com>
Date:   Wed, 10 Nov 2021 23:44:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of
 make_spte

On 11/10/21 23:30, Ben Gardon wrote:
> -	WARN_ONCE(is_rsvd_spte(&vcpu->arch.mmu->shadow_zero_check, spte, level),
> +	WARN_ONCE(is_rsvd_spte(shadow_zero_check, spte, level),
>   		  "spte = 0x%llx, level = %d, rsvd bits = 0x%llx", spte, level,
> -		  get_rsvd_bits(&vcpu->arch.mmu->shadow_zero_check, spte, level));
> +		  get_rsvd_bits(shadow_zero_check, spte, level));

Hmm, there is a deeper issue here, in that when using EPT/NPT (on either 
the legacy aka shadow or the TDP MMU) large parts of vcpu->arch.mmu are 
really the same for all vCPUs.  The only thing that varies is those 
parts that actually depend on the guest's paging mode---the extended 
role, the reserved bits, etc.  Those are needed by the emulator, but 
don't really belong in vcpu->arch.mmu when EPT/NPT is in use.

I wonder if there's room for splitting kvm_mmu in two parts, such as 
kvm_mmu and kvm_guest_paging_context, and possibly change the walk_mmu 
pointer into a pointer to kvm_guest_paging_context.  This way the 
EPT/NPT MMU (again either shadow or TDP) can be moved to kvm->arch.  It 
should simplify this series and also David's work on eager page splitting.

I'm not asking you to do this, of course, but perhaps I can trigger 
Sean's itch to refactor stuff. :)

Paolo