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

Wei Huang
2021-08-08T17:49:36Z

On 8/6/21 12:58 PM, Sean Christopherson wrote:
> On Thu, Aug 05, 2021, Wei Huang wrote:
>> 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 ;-)
>

Thanks for the review. This part of code is indeed subtle. The chaining
trick will be easier to understand with a proper explanation. My
proposal is to keep the original approach, but add more comments to this
group of code.

/*

* Depending on the shadow_root_level, build the root_hpa table
by

* chaining either pml5->pml4->pae or pml4->pae.

*/
mmu->root_hpa = __pa(mmu->pae_root);
if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL) {
mmu->pml4_root[0] = mmu->root_hpa | pm_mask;
mmu->root_hpa = __pa(mmu->pml4_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);
}

This code will be easy to extend for 6-level page table (if needed) in
the future.

>>
>> 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 :-)

You are right that this can be removed.

>
>> 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.

Will do

>
>> + 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.
>

I will take the advice for this part of code.

>> +
>> + 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
>>