On Thu, Apr 14, 2022, Lai Jiangshan wrote: > 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. Hmm, I believe I was thinking that each vCPU could have a pre-allocated pae_root shadow page, i.e. keep pae_root, but make it struct kvm_mmu_page pae_root; The alloc/free paths would still need special handling, but at least in theory, all other code that expects root a shadow page will Just Work.
