Re: [PATCH 1/2] KVM: arm64: Support FFA_MSG_SEND_DIRECT_REQ in host handler

[Per Larsen]

Hi Yeoreum,

On 10/30/25 2:48 PM, Yeoreum Yun wrote:
> Hi Per and Sebasian,
>
> > Allow direct messages to be forwarded from the host.
> >
> > Signed-off-by: Sebastian Ene <sebastianene@xxxxxxxxxx>
> > Signed-off-by: Per Larsen <perlarsen@xxxxxxxxxx>
> > ---
> >   arch/arm64/kvm/hyp/nvhe/ffa.c | 16 ++++++++++++++++
> >   1 file changed, 16 insertions(+)
> >
> > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > index 4e16f9b96f637599873b16148c6e40cf1210aa3e..191dcb301cca3986758fb6a49f15f1799de9f1d1 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > @@ -857,6 +857,15 @@ static void do_ffa_part_get(struct arm_smccc_1_2_regs *res,
> >   	hyp_spin_unlock(&host_buffers.lock);
> >   }
> >
> > +static void do_ffa_direct_msg(struct arm_smccc_1_2_regs *res,
> > +			      struct kvm_cpu_context *ctxt,
> > +			      u64 vm_handle)
> > +{
> > +	struct arm_smccc_1_2_regs *args = (void *)&ctxt->regs.regs[0];
> > +
> > +	arm_smccc_1_2_smc(args, res);
> > +}
> > +
>
> TBH, I don't have a strong comment for this but, I'm not sure why
> it is necessary.
> Since it calls just "smc" with the passed argments,
> I think it can be handled by default_smc_handler() without adding this
> function but return the ture for DIRECT MSG2 in ffa_call_support().
>
> Am I missing something?
Calling `do_ffa_direct_msg` from the host ffa proxy ensures that the 
caller has negotiated a FF-A version with the hypervisor first. In turn,
this means that `ffa_call_support` can use the negotiated version to 
decide whether to proxy this interface or not.

Moreover, `kvm_host_ffa_handler` currently proxies host FF-A calls. 
Android also proxies FF-A calls from guest VMs via a similar function: 
`kvm_guest_ffa_handler` so this function avoids duplication if/when 
adding a guest proxy. This function is also where one would check FFA 
IDs before forwarding messages (to prevent spoofing). You can see the 
downstream implementation here 
https://android-review.googlesource.com/c/kernel/common/+/3422040.

> >   bool kvm_host_ffa_handler(struct kvm_cpu_context *host_ctxt, u32 func_id)
> >   {
> >   	struct arm_smccc_1_2_regs res;
> > @@ -915,6 +924,13 @@ bool kvm_host_ffa_handler(struct kvm_cpu_context *host_ctxt, u32 func_id)
> >   	case FFA_PARTITION_INFO_GET:
> >   		do_ffa_part_get(&res, host_ctxt);
> >   		goto out_handled;
> > +	case FFA_ID_GET:
> > +		ffa_to_smccc_res_prop(&res, FFA_RET_SUCCESS, HOST_FFA_ID);
> > +		goto out_handled;
>
> I think FFA_ID_GET should be a seperated patch?
Agreed. I've dropped it from this patch set as I don't think we need it.>
> > +	case FFA_MSG_SEND_DIRECT_REQ:
> > +	case FFA_FN64_MSG_SEND_DIRECT_REQ:
> > +		do_ffa_direct_msg(&res, host_ctxt, HOST_FFA_ID);
> > +		goto out_handled;
> >   	}
> >
> >   	if (ffa_call_supported(func_id))

Thanks,Per