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

[Jens Wiklander <jens.wiklander@xxxxxxxxxx>]

Hi Bertrand,

On Wed, Feb 11, 2026 at 12:27 PM Bertrand Marquis
<Bertrand.Marquis@xxxxxxx> wrote:
>
> 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.

You're right.

>
> 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 ?

The 1.2 spec only says this w2 is MBZ for Feature IDs, and that w2 is
SBZ for FFA_RXTX_MAP. The 1.3 spec says the same, except that in Table
13.14: Feature IDs and properties table, it lists w2 as SBZ.

Note that FFA_MEM_RETRIEVE_REQ has bits defined in w2, and the unknown
bits are SBZ.

Based on that, I'm inclined to keep it simple and ignore w2-w7.

Cheers,
Jens