Re: [PATCH V3 11/12] KVM: X86/MMU: Don't use mmu->pae_root when shadowing PAE NPT in 64-bit host

Sean Christopherson
2022-07-19T23:26:15Z

On Sat, May 21, 2022, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
>
> Allocate the tables when allocating the local shadow page.

This absolutely needs a much more verbose changelog.

> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 17 +++++++++--------
> 1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 63c2b2c6122c..73e6a8e1e1a9 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1809,10 +1809,12 @@ static bool using_local_root_page(struct kvm_mmu *mmu)
> * 2 or 3 levels of local shadow pages on top of non-local shadow pages.
> *
> * Local shadow pages are locally allocated. If the local shadow page's level
> - * is PT32E_ROOT_LEVEL, it will use the preallocated mmu->pae_root for its
> - * sp->spt. Because sp->spt may need to be put in the 32 bits CR3 (even in
> - * x86_64) or decrypted. Using the preallocated one to handle these
> - * requirements makes the allocation simpler.
> + * is PT32E_ROOT_LEVEL, and it is not shadowing nested NPT for 32-bit L1 in
> + * 64-bit L0 (or said when the shadow pagetable's level is PT32E_ROOT_LEVEL),
> + * it will use the preallocated mmu->pae_root for its sp->spt. Because sp->spt
> + * need to be put in the 32-bit CR3 (even in 64-bit host) or decrypted. Using
> + * the preallocated one to handle these requirements makes the allocation
> + * simpler.
> *
> * Local shadow pages are only visible to local VCPU except through
> * sp->parent_ptes rmap from their children, so they are not in the
> @@ -1852,13 +1854,12 @@ kvm_mmu_alloc_local_shadow_page(struct kvm_vcpu *vcpu, union kvm_mmu_page_role r
> sp->gfn = 0;
> sp->role = role;
> /*
> - * Use the preallocated mmu->pae_root when the shadow page's
> - * level is PT32E_ROOT_LEVEL which may need to be put in the 32 bits
> + * Use the preallocated mmu->pae_root when the shadow pagetable's
> + * level is PT32E_ROOT_LEVEL which need to be put in the 32 bits
> * CR3 (even in x86_64) or decrypted. The preallocated one is prepared
> * for the requirements.
> */
> - if (role.level == PT32E_ROOT_LEVEL &&
> - !WARN_ON_ONCE(!vcpu->arch.mmu->pae_root))

Why remove this WARN_ON_ONCE()? And shouldn't this also interact with

KVM: X86/MMU: Allocate mmu->pae_root for PAE paging on-demand

Actually, I think the series is buggy. That patch, which precedes this one, does

if (vcpu->arch.mmu->root_role.level != PT32E_ROOT_LEVEL)
return 0;

i.e. does NOT allocate pae_root for a 64-bit host, which means that running KVM
against the on-demand patch would result in the WARN firing and bad things happening.

> + if (vcpu->arch.mmu->root_role.level == PT32E_ROOT_LEVEL)
> sp->spt = vcpu->arch.mmu->pae_root;
> else
> sp->spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache);
> --
> 2.19.1.6.gb485710b
>