Re: [PATCH v4 3/3] KVM: arm64: Release the ownership of the hyp rx buffer to Trustzone

[Quentin Perret <qperret@google.com>]

On Wednesday 26 Mar 2025 at 11:39:01 (+0000), Sebastian Ene wrote:
> Introduce the release FF-A call to notify Trustzone that the hypervisor
> has finished copying the data from the buffer shared with Trustzone to
> the non-secure partition.
>
> Reported-by: Andrei Homescu <ahomescu@google.com>
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> ---
>  arch/arm64/kvm/hyp/nvhe/ffa.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index 6df6131f1107..ac898ea6274a 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -749,6 +749,7 @@ static void do_ffa_part_get(struct arm_smccc_res *res,
>  	DECLARE_REG(u32, uuid3, ctxt, 4);
>  	DECLARE_REG(u32, flags, ctxt, 5);
>  	u32 count, partition_sz, copy_sz;
> +	struct arm_smccc_res _res;
>  
>  	hyp_spin_lock(&host_buffers.lock);
>  	if (!host_buffers.rx) {
> @@ -765,11 +766,11 @@ static void do_ffa_part_get(struct arm_smccc_res *res,
>  
>  	count = res->a2;
>  	if (!count)
> -		goto out_unlock;
> +		goto release_rx;
>  
>  	if (hyp_ffa_version > FFA_VERSION_1_0) {
>  		/* Get the number of partitions deployed in the system */
> -		if (flags & 0x1)
> +		if (flags & PARTITION_INFO_GET_RETURN_COUNT_ONLY)
>  			goto out_unlock;
>  
>  		partition_sz  = res->a3;
> @@ -781,10 +782,12 @@ static void do_ffa_part_get(struct arm_smccc_res *res,
>  	copy_sz = partition_sz * count;
>  	if (copy_sz > KVM_FFA_MBOX_NR_PAGES * PAGE_SIZE) {
>  		ffa_to_smccc_res(res, FFA_RET_ABORTED);
> -		goto out_unlock;
> +		goto release_rx;
>  	}
>  
>  	memcpy(host_buffers.rx, hyp_buffers.rx, copy_sz);
> +release_rx:
> +	ffa_rx_release(&_res);

I'm a bit confused about this release call here. In the pKVM FF-A proxy
model, the hypervisor is essentially 'transparent', so do we not expect
EL1 to issue that instead? How is EL1 supposed to know that the
hypervisor has already sent the release call? And isn't EL1 going to be
confused if the content of the buffer is overridden before is has issued
the release call itself? What would otherwise prevent that from
happening?

Thanks,
Quentin

>  out_unlock:
>  	hyp_spin_unlock(&host_buffers.lock);
>  }
> -- 
> 2.49.0.395.g12beb8f557-goog
>