linux-kernel - Re: [RFC PATCH V2 0/4] KVM: X86: Add and use shadow page with level expanded or acting as pae

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Date:   Wed, 30 Mar 2022 07:41:39 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshan.ljs@...group.com>
Subject: Re: [RFC PATCH V2 0/4] KVM: X86: Add and use shadow page with level
 expanded or acting as pae_root

On Wed, Mar 30, 2022 at 4:31 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 3/29/22 17:36, Lai Jiangshan wrote:
> > From: Lai Jiangshan <jiangshan.ljs@...group.com>
> >
> > (Request For Help for testing on AMD machine with 32 bit L1 hypervisor,
> > see information below)
> >
> > KVM handles root pages specially for these cases:
> >
> > direct mmu (nonpaping for 32 bit guest):
> >       gCR0_PG=0
> > shadow mmu (shadow paping for 32 bit guest):
> >       gCR0_PG=1,gEFER_LMA=0,gCR4_PSE=0
> >       gCR0_PG=1,gEFER_LMA=0,gCR4_PSE=1
> > direct mmu (NPT for 32bit host):
> >       hEFER_LMA=0
> > shadow nested NPT (for 32bit L1 hypervisor):
> >       gCR0_PG=1,gEFER_LMA=0,gCR4_PSE=0,hEFER_LMA=0
> >       gCR0_PG=1,gEFER_LMA=0,gCR4_PSE=1,hEFER_LMA=0
> >       gCR0_PG=1,gEFER_LMA=0,gCR4_PSE={0|1},hEFER_LMA=1,hCR4_LA57={0|1}
> > Shadow nested NPT for 64bit L1 hypervisor:
> >       gEFER_LMA=1,gCR4_LA57=0,hEFER_LMA=1,hCR4_LA57=1
> >
> > They are either using special roots or matched the condition
> > ((mmu->shadow_root_level > mmu->root_level) && !mm->direct_map)
> > (refered as level expansion) or both.
> >
> > All the cases are using special roots except the last one.
> > Many cases are doing level expansion including the last one.
>
> Hi Jiangshan,
>
> so the main difference between direct and passthrough shadow pages is
> that passthrough pages can have indirect children.  A direct page maps
> the page at sp->gfn, while a passthrough page maps the page _table_ at
> sp->gfn.  Is this correct?
>
> If so, I think there is a difference between a passthrough page that
> maps a level-2 page from level-4, and a passthrough page that maps a
> level-3 page from level-4.  If that is true, a single bit in the role
> is not enough.

Ahhhh, you are correct.

>
> One way to handle this could be to have a single field "mapping_level"
> that subsumes both "direct" and "passthrough".  direct==1
> would correspond to "mapping_level == 0"; direct==0 && passthrough==0
> would be "mapping_level == level"; anything in the middle would be a
> passthrough page in your series.
>
> What do you think?
>
> Thanks,
>
> Paolo
>