Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of make_spte

From: Ben Gardon <bgardon@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	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>
Subject: Re: [RFC 11/19] KVM: x86/mmu: Factor shadow_zero_check out of make_spte
Date: Wed, 10 Nov 2021 15:49:44 -0800	[thread overview]
Message-ID: <CANgfPd8hzDU+v52t9Kr=b48utC1p_j3yJ8gHzo-uifAxHbh-eQ@mail.gmail.com> (raw)
In-Reply-To: <80407e4a-36e1-e606-ed9f-74429f850e77@redhat.com>

On Wed, Nov 10, 2021 at 2:45 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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
>

I think that's a great idea. I'm frequently confused as to why the
struct kvm_mmu is a per-vcpu construct as opposed to being VM-global.
Moving part of the struct to be a member for struct kvm would also
open the door to formalizing the MMU interface a little better and
perhaps even reveal more MMU code that can be consolidated across
architectures.