Re: [PATCH 08/12] xen/arm: ffa: Fix FFA_FEATURES validation

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Jens,

> On 11 Feb 2026, at 09:17, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
> 
> Hi Bertrand,
> 
> On Tue, Feb 3, 2026 at 6:38 PM Bertrand Marquis
> <bertrand.marquis@xxxxxxx> wrote:
>> 
>> FFA_FEATURES currently accepts non-zero input properties (w2-w7) from
>> guests and advertises several ABIs unconditionally, even when firmware
>> support is missing or when the ABI is physical-instance-only. This can
>> mislead guests about what Xen can actually provide and violates FF-A
>> calling conventions. Some SPMCs (Hafnium v2.14 or earlier) also fail to
>> report FFA_RX_ACQUIRE despite supporting it.
>> 
>> Update FFA_FEATURES validation to match spec and firmware support:
>> - reject non-zero w2-w7 input properties with INVALID_PARAMETERS
>> - reject 64-bit calling conventions from 32-bit guests with NOT_SUPPORTED
>> - return NOT_SUPPORTED for physical-instance-only ABIs
>> (FFA_NOTIFICATION_BITMAP_{CREATE,DESTROY}, FFA_RX_ACQUIRE)
>> - advertise FFA_INTERRUPT as supported
>> - gate message ABIs on firmware support:
>> - FFA_MSG_SEND_DIRECT_REQ_{32,64}
>> - FFA_MSG_SEND_DIRECT_REQ2 (also requires FF-A 1.2 negotiation)
>> - FFA_MSG_SEND2 (or VM-to-VM enabled)
>> - report MEM_SHARE_{32,64} only when FFA_MEM_SHARE_64 is supported
>> - stop advertising FFA_MSG_YIELD (not implemented)
>> 
>> Update firmware probing: drop FFA_MEM_SHARE_32 checks (deprecated) and
>> add FFA_RX_ACQUIRE to the probed set. If FFA_MSG_SEND2 is reported but
>> FFA_RX_ACQUIRE is not, assume RX_ACQUIRE support and warn to work
>> around the Hafnium bug.
>> 
>> Functional impact: guests now see ABI support that reflects firmware
>> capabilities and Xen implementation status. When SEND2 is present but
>> RX_ACQUIRE is not reported, Xen assumes RX_ACQUIRE support.
>> 
>> Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>> ---
>> xen/arch/arm/tee/ffa.c | 62 +++++++++++++++++++++++++++++++++++++-----
>> 1 file changed, 55 insertions(+), 7 deletions(-)
>> 
>> diff --git a/xen/arch/arm/tee/ffa.c b/xen/arch/arm/tee/ffa.c
>> index 6de2b9f8ac8e..e9e020bb0cb3 100644
>> --- a/xen/arch/arm/tee/ffa.c
>> +++ b/xen/arch/arm/tee/ffa.c
>> @@ -91,10 +91,10 @@ static const struct ffa_fw_abi ffa_fw_abi_needed[] = {
>>     FW_ABI(FFA_PARTITION_INFO_GET),
>>     FW_ABI(FFA_NOTIFICATION_INFO_GET_64),
>>     FW_ABI(FFA_NOTIFICATION_GET),
>> +    FW_ABI(FFA_RX_ACQUIRE),
>>     FW_ABI(FFA_RX_RELEASE),
>>     FW_ABI(FFA_RXTX_MAP_64),
>>     FW_ABI(FFA_RXTX_UNMAP),
>> -    FW_ABI(FFA_MEM_SHARE_32),
>>     FW_ABI(FFA_MEM_SHARE_64),
>>     FW_ABI(FFA_MEM_RECLAIM),
>>     FW_ABI(FFA_MSG_SEND_DIRECT_REQ_32),
>> @@ -240,19 +240,39 @@ static void handle_features(struct cpu_user_regs *regs)
>>     struct ffa_ctx *ctx = d->arch.tee;
>>     unsigned int n;
>> 
>> +    /*
>> +     * Xen does not accept any non-zero FFA_FEATURES input properties from
>> +     * VMs. The spec only defines w2 input properties for 
>> FFA_MEM_RETRIEVE_REQ
>> +     * (NS-bit negotiation for SP/SPMC) and FFA_RXTX_MAP (buffer size and
>> +     * alignment), so w2 must be MBZ for our callers.
>> +     */
> 
> The spec (version 1.2) lists them as SBZ, except for w2, which is MBZ,
> for Feature IDs.

Very true, this should only check w2 which is anyway defined as MBZ when
not used.
w3-w7 were MBZ in previous versions of FF-A but are in fact SBZ in 1.2 so
we should ignore them

> However, if we're to return an error, invalid parameters is a better choice.

In fact the spec is actually saying the following:
If the FF-A interface or feature that was queried is not implemented or invalid,
the callee completes this call with an invocation of the FFA_ERROR interface
with the NOT_SUPPORTED error code.

So there is no case for INVALID_PARAMETER.

So in fact i should:
- return NOT_SUPPORTED if w2 is not 0
- ignore w3-w7

Can you confirm that you have the same reading of the spec than me ?

Cheers
Bertrand