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

[Marc Zyngier <maz@kernel.org>]

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

-- 
Without deviation from the norm, progress is not possible.