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

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Date:   Thu, 11 Nov 2021 08:06:31 +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 11/11/21 02:18, Sean Christopherson wrote:
> But what would you actually move?  Even shadow_zero_check barely squeaks by,
> e.g. if NX is ever used to for NPT, then maybe it stops being a per-VM setting.

Hmm, I think it would still be per-VM, just like 32-bit shadow page tables
are always built for EFER.NXE=CR4.PAE=1.  Anyway, the rough sketch is to have
three structs:

* struct kvm_mmu_kind has the function pointers and the state that is
needed to operate on page tables

* struct kvm_mmu has the function pointers and the state that is
needed while the vCPU runs, including the role

* struct kvm_paging_context has the stuff related to emulation;
shadow page tables of course needs it but EPT/NPT do not (with
either the old or the new MMU)

So you'd have a "struct kvm_mmu_kind direct_mmu" in struct kvm_arch (for
either legacy EPT/NPT or the new MMU), and

	struct kvm_mmu_kind shadow_mmu;
	struct kvm_mmu root_mmu;		/* either TDP or shadow */
	struct kvm_mmu tdp12_mmu;		/* always shadow */
	struct kvm_mmu *mmu;			/* either &kvm->direct_mmu or &vcpu->shadow_mmu */
	struct kvm_paging_context root_walk;	/* maybe unified with walk01 below? dunno yet */
	struct kvm_paging_context walk01;
	struct kvm_paging_context walk12;
	struct kvm_paging_context *walk;	/* either &vcpu->root_walk or &vcpu->walk12 */

in struct kvm_vcpu_arch.  struct kvm_mmu* has a pointer to
struct kvm_mmu_kind*; however, if an spte.c function does not need
the data in struct kvm_mmu_state*, it can take a struct kvm_mmu_kind*
and it won't need a vCPU.  Likewise the TDP MMU knows its kvm_mmu_kind
is always in &kvm->direct_mmu so it can take a struct kvm* if the struct
kvm_mmu_state* is not needed.

The first part of the refactoring would be to kill the nested_mmu
and walk_mmu, replacing them by &vcpu->walk12 and vcpu->walk
respectively.  The half-useless nested_mmu has always bothered me,
I was going to play with it anyway because I want to remove the
kvm_mmu_reset_context from CR0.WP writes, I'll see if I get
something useful out of it.

Paolo