linux-kernel - Re: [PATCH 1/2] KVM: arm64: Support FFA_MSG_SEND_DIRECT

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Message-ID: <fb69e006-8460-4a5d-a19e-28bdec10434d@immunant.com>
Date: Thu, 30 Oct 2025 17:18:51 +0100
From: Per Larsen <perl@...unant.com>
To: Yeoreum Yun <yeoreum.yun@....com>, perlarsen@...gle.com
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Joey Gouly <joey.gouly@....com>, Suzuki K Poulose <suzuki.poulose@....com>,
 Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ben Horgan <ben.horgan@....com>, Armelle Laine <armellel@...gle.com>,
 Sebastien Ene <sebastianene@...gle.com>,
 linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: arm64: Support FFA_MSG_SEND_DIRECT_REQ in host
 handler

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@...gle.com>
>> Signed-off-by: Per Larsen <perlarsen@...gle.com>
>> ---
>>   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