Re: [PATCH v1 2/3] KVM: x86: Handle the case of 5-level shadow page table

Sean Christopherson
2021-08-06T17:58:38Z

On Thu, Aug 05, 2021, Wei Huang wrote:
> When the 5-level page table CPU flag is exposed, KVM code needs to handle
> this case by pointing mmu->root_hpa to a properly-constructed 5-level page
> table.
>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Wei Huang <wei.huang2@amd.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/mmu/mmu.c | 46 +++++++++++++++++++++++----------
> 2 files changed, 34 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 20ddfbac966e..8586ffdf4de8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -447,6 +447,7 @@ struct kvm_mmu {
>
> u64 *pae_root;
> u64 *pml4_root;
> + u64 *pml5_root;
>
> /*
> * check zero bits on shadow page table entries, these
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 44e4561e41f5..b162c3e530aa 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3428,7 +3428,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> * the shadow page table may be a PAE or a long mode page table.
> */
> pm_mask = PT_PRESENT_MASK | shadow_me_mask;
> - if (mmu->shadow_root_level == PT64_ROOT_4LEVEL) {
> + if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
> pm_mask |= PT_ACCESSED_MASK | PT_WRITABLE_MASK | PT_USER_MASK;
>
> if (WARN_ON_ONCE(!mmu->pml4_root)) {
> @@ -3454,11 +3454,17 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> PT32_ROOT_LEVEL, false);
> mmu->pae_root[i] = root | pm_mask;
> }
> + mmu->root_hpa = __pa(mmu->pae_root);
>
> - if (mmu->shadow_root_level == PT64_ROOT_4LEVEL)
> + if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
> + mmu->pml4_root[0] = mmu->root_hpa | pm_mask;
> mmu->root_hpa = __pa(mmu->pml4_root);
> - else
> - mmu->root_hpa = __pa(mmu->pae_root);
> + }
> +
> + if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
> + mmu->pml5_root[0] = mmu->root_hpa | pm_mask;
> + mmu->root_hpa = __pa(mmu->pml5_root);
> + }

Ouch, the root_hpa chaining is subtle. That's my fault :-) I think it would be
better to explicitly chain pae->pml4->pml5? E.g.

if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
mmu->pml4_root[0] = __pa(mmu->pae_root) | pm_mask;

if (mmu->shadow_root_level == PT64_ROOT_5LEVEL) {
mmu->pml5_root[0] = __pa(mmu->pml4_root) | pm_mask;
mmu->root_hpa = __pa(mmu->pml5_root);
} else {
mmu->root_hpa = __pa(mmu->pml4_root);
}
} else {
mmu->root_hpa = __pa(mmu->pae_root);
}

It'd require more churn if we get to 6-level paging, but that's a risk I'm willing
to take ;-)

>
> set_root_pgd:
> mmu->root_pgd = root_pgd;
> @@ -3471,7 +3477,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
> static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
> {
> struct kvm_mmu *mmu = vcpu->arch.mmu;
> - u64 *pml4_root, *pae_root;
> + u64 *pml5_root, *pml4_root, *pae_root;
>
> /*
> * When shadowing 32-bit or PAE NPT with 64-bit NPT, the PML4 and PDP
> @@ -3487,17 +3493,18 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
> * This mess only works with 4-level paging and needs to be updated to
> * work with 5-level paging.
> */
> - if (WARN_ON_ONCE(mmu->shadow_root_level != PT64_ROOT_4LEVEL))
> + if (WARN_ON_ONCE(mmu->shadow_root_level < PT64_ROOT_4LEVEL)) {

This is amusingly wrong. The check above this is:

if (mmu->direct_map || mmu->root_level >= PT64_ROOT_4LEVEL ||
mmu->shadow_root_level < PT64_ROOT_4LEVEL) <--------
return 0;

meaning this is dead code. It should simply deleted. If we reaaaaaly wanted to
future proof the code, we could do:

if (WARN_ON_ONCE(mmu->shadow_root_level > PT64_ROOT_5LEVEL)
return -EIO;

but at that point we're looking at a completely different architecture, so I don't
think we need to be that paranoid :-)

> return -EIO;
> + }
>
> - if (mmu->pae_root && mmu->pml4_root)
> + if (mmu->pae_root && mmu->pml4_root && mmu->pml5_root)
> return 0;
>
> /*
> * The special roots should always be allocated in concert. Yell and
> * bail if KVM ends up in a state where only one of the roots is valid.
> */
> - if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root))
> + if (WARN_ON_ONCE(!tdp_enabled || mmu->pae_root || mmu->pml4_root || mmu->pml5_root))
> return -EIO;
>
> /*
> @@ -3506,18 +3513,30 @@ static int mmu_alloc_special_roots(struct kvm_vcpu *vcpu)
> */
> pae_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> if (!pae_root)
> - return -ENOMEM;
> + goto err_out;

Branching to the error handling here is silly, it's the first allocation.

>
> pml4_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
> - if (!pml4_root) {
> - free_page((unsigned long)pae_root);
> - return -ENOMEM;
> - }
> + if (!pml4_root)
> + goto err_out;
> +
> + pml5_root = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);

This should be guarded by "mmu->shadow_root_level > PT64_ROOT_4LEVEL", there's no
need to waste a page on PML5 if it can't exist.

> + if (!pml5_root)
> + goto err_out;
>
> mmu->pae_root = pae_root;
> mmu->pml4_root = pml4_root;
> + mmu->pml5_root = pml5_root;
>
> return 0;
> +err_out:
> + if (pae_root)
> + free_page((unsigned long)pae_root);
> + if (pml4_root)
> + free_page((unsigned long)pml4_root);
> + if (pml5_root)
> + free_page((unsigned long)pml5_root);

This is flawed as failure to allocate pml4_root will consume an uninitialized
pml5_root. There's also no need to check for non-NULL values as free_page plays
nice with NULL pointers.

If you drop the unnecessary goto for pae_root allocation failure, than this can
become:

err_out:
free_page((unsigned long)pml4_root);
free_page((unsigned long)pae_root);

since pml4_root will be NULL if pml4_root allocation failures. IMO that's
unnecessarily clever though, and a more standard:

err_pml5:
free_page((unsigned long)pml4_root);
err_pml4:
free_page((unsigned long)pae_root);
return -ENOMEM;

would be far easier to read/maintain.

> +
> + return -ENOMEM;
> }
>
> void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> @@ -5320,6 +5339,7 @@ static void free_mmu_pages(struct kvm_mmu *mmu)
> set_memory_encrypted((unsigned long)mmu->pae_root, 1);
> free_page((unsigned long)mmu->pae_root);
> free_page((unsigned long)mmu->pml4_root);
> + free_page((unsigned long)mmu->pml5_root);
> }
>
> static int __kvm_mmu_create(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
> --
> 2.31.1
>