Re: [PATCH v3 15/15] KVM: arm64: pkvm: Unshare guest structs during teardown

[Will Deacon <will@kernel.org>]

On Wed, Dec 01, 2021 at 05:04:09PM +0000, Quentin Perret wrote:
> Make use of the newly introduced unshare hypercall during guest teardown
> to unmap guest-related data structures from the hyp stage-1.
> 
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  2 ++
>  arch/arm64/include/asm/kvm_mmu.h  |  1 +
>  arch/arm64/kvm/arm.c              |  2 ++
>  arch/arm64/kvm/fpsimd.c           | 34 ++++++++++++++++++++++---
>  arch/arm64/kvm/mmu.c              | 42 +++++++++++++++++++++++++++++++
>  arch/arm64/kvm/reset.c            |  8 +++++-
>  6 files changed, 85 insertions(+), 4 deletions(-)

[...]

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index d72566896755..8e506ba8988e 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -344,6 +344,32 @@ static int share_pfn_hyp(u64 pfn)
>  	return ret;
>  }
>  
> +static int unshare_pfn_hyp(u64 pfn)
> +{
> +	struct rb_node **node, *parent;
> +	struct hyp_shared_pfn *this;
> +	int ret = 0;
> +
> +	mutex_lock(&hyp_shared_pfns_lock);
> +	this = find_shared_pfn(pfn, &node, &parent);
> +	if (WARN_ON(!this)) {
> +		ret = -EINVAL;

-ENOENT?

> +		goto unlock;
> +	}
> +
> +	this->count--;
> +	if (this->count)
> +		goto unlock;

Again, if we did an RCU lookup then this could be converted to a refcount_t
to take the mutex only when it hits zero. But for now I think it's fine.

> +
> +	rb_erase(&this->node, &hyp_shared_pfns);
> +	kfree(this);
> +	ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn, 1);
> +unlock:
> +	mutex_unlock(&hyp_shared_pfns_lock);
> +
> +	return ret;
> +}
> +
>  int kvm_share_hyp(void *from, void *to)
>  {
>  	phys_addr_t start, end, cur;
> @@ -376,6 +402,22 @@ int kvm_share_hyp(void *from, void *to)
>  	return 0;
>  }
>  
> +void kvm_unshare_hyp(void *from, void *to)
> +{
> +	phys_addr_t start, end, cur;
> +	u64 pfn;
> +
> +	if (is_kernel_in_hyp_mode() || kvm_host_owns_hyp_mappings() || !from)

I don't think you need the is_kernel_in_hyp_mode() check any more not that
you've moved that into kvm_host_owns_hyp_mappings().

Will