Message-ID: <80407e4a-36e1-e606-ed9f-74429f850e77@redhat.com>
Date:   Wed, 10 Nov 2021 23:44:54 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.2.0
Subject: Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of
 make_spte
Content-Language: en-US
To:     Ben Gardon <bgardon@google.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Peter Xu <peterx@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Peter Shier <pshier@google.com>,
        David Matlack <dmatlack@google.com>,
        Mingwei Zhang <mizhang@google.com>,
        Yulei Zhang <yulei.kernel@gmail.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        Xiao Guangrong <xiaoguangrong.eric@gmail.com>,
        Kai Huang <kai.huang@intel.com>,
        Keqian Zhu <zhukeqian1@huawei.com>,
        David Hildenbrand <david@redhat.com>
References: <20211110223010.1392399-1-bgardon@google.com>
 <20211110223010.1392399-12-bgardon@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20211110223010.1392399-12-bgardon@google.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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