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

[Marc Zyngier <maz@kernel.org>]

On Fri, 26 Nov 2021 05:59:00 +0000,
Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> 
> 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.

Yes. I have added something along those lines.

> > +		 */
> > +		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>

Given that this was a fixup, I haven't taken this tag. I will Cc you
on the whole series, and you can give you tag on the whole patch if
you are happy with it.

BTW, I have now fixed the bug that was preventing L2 userspace from
running (bad interaction with the pgtable code which was unhappy about
my use of the SW bits when relaxing the permissions). You should now
be able to test the whole series.

Thanks,

	M.

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