Re: [RFC PATCH V3 3/4] KVM: X86: Alloc role.pae_root shadow page

[Lai Jiangshan]

On Wed, Apr 13, 2022 at 5:14 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Mar 30, 2022, Lai Jiangshan wrote:
> > From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
> >
> > Currently pae_root is special root page, this patch adds facility to
> > allow using kvm_mmu_get_page() to allocate pae_root shadow page.
>
> I don't think this will work for shadow paging.  CR3 only has to be 32-byte aligned
> for PAE paging.  Unless I'm missing something subtle in the code, KVM will incorrectly
> reuse a pae_root if the guest puts multiple PAE CR3s on a single page because KVM's
> gfn calculation will drop bits 11:5.

I forgot about it.

>
> Handling this as a one-off is probably easier.  For TDP, only 32-bit KVM with NPT
> benefits from reusing roots, IMO and shaving a few pages in that case is not worth
> the complexity.
>

I liked the one-off idea yesterday and started trying it.

But things were not going as smoothly as I thought.  There are too
many corner cases to cover.  Maybe I don't get what you envisioned.

one-off shadow pages must not be in the hash, must be freed
immediately in kvm_mmu_free_roots(), taken care in
kvm_mmu_prepare_zap_page() and so on.

When the guest is 32bit, the host has to free and allocate sp
every time when the guest changes cr3.  It will be a regression
when !TDP.

one-off shadow pages are too distinguished from others.

When using one-off shadow pages, role.passthough can be one
bit and be used only for 5-level NPT L0 for 4-level NPT L1,
which is neat.  And role.pae_root can be removed.

I want the newly added shadow pages to fit into the current
shadow page management and root management.

I'm going to add sp->pae_off (u16) which is 11:5 of the cr3
when the guest is PAE paging. It needs only less than 10 lines
of code.

Thanks.
Lai