linux-kernel - Re: [PATCH V3 02/12] KVM: X86/MMU: Add using_local_root

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Message-ID: <Yo/za2jnWUHvoLQg@google.com>
Date:   Thu, 26 May 2022 21:38:51 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>
Subject: Re: [PATCH V3 02/12] KVM: X86/MMU: Add using_local_root_page()

On Thu, May 26, 2022, David Matlack wrote:
> On Sat, May 21, 2022 at 09:16:50PM +0800, Lai Jiangshan wrote:
> > From: Lai Jiangshan <jiangshan.ljs@...group.com>
> > 
> > In some cases, local root pages are used for MMU.  It is often using
> > to_shadow_page(mmu->root.hpa) to check if local root pages are used.
> > 
> > Add using_local_root_page() to directly check if local root pages are
> > used or needed to be used even mmu->root.hpa is not set.
> > 
> > Prepare for making to_shadow_page(mmu->root.hpa) returns non-NULL via
> > using local shadow [root] pages.
> > 
> > Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 40 +++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 37 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index efe5a3dca1e0..624b6d2473f7 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1690,6 +1690,39 @@ static void drop_parent_pte(struct kvm_mmu_page *sp,
> >  	mmu_spte_clear_no_track(parent_pte);
> >  }
> >  
> > +/*
> > + * KVM uses the VCPU's local root page (vcpu->mmu->pae_root) when either the
> > + * shadow pagetable is using PAE paging or the host is shadowing nested NPT for
> > + * 32bit L1 hypervisor.
> 
> How about using the terms "private" and "shared" instead of "local" and
> "non-local"? I think that more accurately conveys what is special about
> these pages: they are private to the vCPU using them. And then "shared"
> is more intuitive to understand than "non-local" (which is used
> elsewhere in this series).

Please avoid "private" and "shared".  I haven't read the full context of the
discussion, but those terms have already been claimed by confidential VMs.

FWIW, I believe similar discussions happened around mm/ and kmap(), and they ended
up with thread_local and kmap_local().  Maybe "vCPU local" and "common"?