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

[Wei Huang <wei.huang2@amd.com>]

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
>>