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

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Message-ID: <db8a2431-8a05-bd50-dd79-74c814c71edd@redhat.com>
Date:   Thu, 18 Nov 2021 18:19:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Peter Xu <peterx@...hat.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 11/18/21 17:37, Sean Christopherson wrote:
>> It's a bit ugly in that we'd pass both @kvm and @vcpu, so that needs some more
>> thought, but at minimum it means there's no need to recalc the reserved bits.
>
> Ok, I think my final vote is to have the reserved bits passed in, but with the
> non-nested TDP reserved bits being computed at MMU init.

Yes, and that's also where I was getting with the idea of moving part of 
the "direct" MMU (man, naming these things is so hard) to struct kvm: 
split the per-vCPU state from the constant one and initialize the latter 
just once.  Though perhaps I was putting the cart slightly before the horse.

On the topic of naming, we have a lot of things to name:

- the two MMU codebases: you Googlers are trying to grandfather "legacy" 
and "TDP" into upstream, but that's not a great name because the former 
is used also when shadowing EPT/NPT.  I'm thinking of standardizing on 
"shadow" and "TDP" (it's not perfect because of the 32-bit and tdp_mmu=0 
cases, but it's a start).  Maybe even split parts of mmu.c out into 
shadow_mmu.c.

- the two walkers (I'm quite convinced of splitting that part out of 
struct kvm_mmu and getting rid of walk_mmu/nested_mmu): that's easy, it 
can be walk01 and walk12 with "walk" pointing to one of them

- the two MMUs: with nested_mmu gone, root_mmu and guest_mmu are much 
less confusing and we can keep those names.

Paolo