Re: [PATCH 2/2] KVM: arm64: nv: fixup! Support multiple nested Stage-2 mmu structures

[Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>]

Hi Marc,

On 25-11-2021 07:53 pm, Marc Zyngier wrote:
> On Mon, 22 Nov 2021 09:58:03 +0000,
> Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
>>
>> Commit 1776c91346b6 ("KVM: arm64: nv: Support multiple nested Stage-2 mmu
>> structures")[1] added a function kvm_vcpu_init_nested which expands the
>> stage-2 mmu structures array when ever a new vCPU is created. The array
>> is expanded using krealloc() and results in a stale mmu address pointer
>> in pgt->mmu. Adding a fix to update the pointer with the new address after
>> successful krealloc.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/
>> branch kvm-arm64/nv-5.13
>>
>> Signed-off-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
>> ---
>>   arch/arm64/kvm/nested.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
>> index 4ffbc14d0245..57ad8d8f4ee5 100644
>> --- a/arch/arm64/kvm/nested.c
>> +++ b/arch/arm64/kvm/nested.c
>> @@ -68,6 +68,8 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
>>   		       num_mmus * sizeof(*kvm->arch.nested_mmus),
>>   		       GFP_KERNEL | __GFP_ZERO);
>>   	if (tmp) {
>> +		int i;
>> +
>>   		if (kvm_init_stage2_mmu(kvm, &tmp[num_mmus - 1]) ||
>>   		    kvm_init_stage2_mmu(kvm, &tmp[num_mmus - 2])) {
>>   			kvm_free_stage2_pgd(&tmp[num_mmus - 1]);
>> @@ -80,6 +82,13 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
>>   		}
>>   
>>   		kvm->arch.nested_mmus = tmp;
>> +
>> +		/* Fixup pgt->mmu after krealloc */
>> +		for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
>> +			struct kvm_s2_mmu *mmu = &kvm->arch.nested_mmus[i];
>> +
>> +			mmu->pgt->mmu = mmu;
>> +		}
>>   	}
>>   
>>   	mutex_unlock(&kvm->lock);
> 
> Another good catch. I've tweaked a bit to avoid some unnecessary
> repainting, see below.
> 
> Thanks again,
> 
> 	M.
> 
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index a4dfffa1dae0..92b225db59ac 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -66,8 +66,19 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
>   	num_mmus = atomic_read(&kvm->online_vcpus) * 2;
>   	tmp = krealloc(kvm->arch.nested_mmus,
>   		       num_mmus * sizeof(*kvm->arch.nested_mmus),
> -		       GFP_KERNEL | __GFP_ZERO);
> +		       GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>   	if (tmp) {
> +		/*
> +		 * If we went through a realocation, adjust the MMU

Is it more precise to say?
> +		 * back-pointers in the pg_table structures.
* back-pointers in the pg_table structures of previous inits.

> +		 */
> +		if (kvm->arch.nested_mmus != tmp) {
> +			int i;
> +
> +			for (i = 0; i < num_mms - 2; i++)
> +				tmp[i].pgt->mmu = &tmp[i];
> +		}

Thanks for this optimization, it saves 2 redundant iterations.
> +
>   		if (kvm_init_stage2_mmu(kvm, &tmp[num_mmus - 1]) ||
>   		    kvm_init_stage2_mmu(kvm, &tmp[num_mmus - 2])) {
>   			kvm_free_stage2_pgd(&tmp[num_mmus - 1]);
> 

Feel free to add,
Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>


Thanks,
Ganapat