Re: [XEN PATCH v7 10/20] xen/arm: ffa: add direct request support

[Jens Wiklander <jens.wiklander@xxxxxxxxxx>]

Hi Bertrand,

On Mon, Feb 27, 2023 at 4:28 PM Bertrand Marquis
<Bertrand.Marquis@xxxxxxx> wrote:
>
> Hi Jens,
>
> > On 22 Feb 2023, at 16:33, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
> >
> > Adds support for sending a FF-A direct request. Checks that the SP also
> > supports handling a 32-bit direct request. 64-bit direct requests are
> > not used by the mediator itself so there is not need to check for that.
> >
> > Signed-off-by: Jens Wiklander <jens.wiklander@xxxxxxxxxx>
> > ---
> > xen/arch/arm/tee/ffa.c | 119 +++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 119 insertions(+)
> >
> > diff --git a/xen/arch/arm/tee/ffa.c b/xen/arch/arm/tee/ffa.c
> > index 463fd7730573..a5d8a12635b6 100644
> > --- a/xen/arch/arm/tee/ffa.c
> > +++ b/xen/arch/arm/tee/ffa.c
> > @@ -142,6 +142,7 @@
> >
> > struct ffa_ctx {
> >     uint32_t guest_vers;
> > +    bool interrupted;
>
> This is added and set here for one special error code but is never used.
> I would suggest to introduce this when there will be an action based on it.

I'm sorry, I forgot about completing this. I'll add code to deal with
FFA_INTERRUPT. This will be tricky to test though since we don't use
FFA_INTERRUPT like this with OP-TEE. The Hypervisor is required by the
FF-A standard to support it so I better add something.

>
> > };
> >
> > /* Negotiated FF-A version to use with the SPMC */
> > @@ -167,6 +168,55 @@ static bool ffa_get_version(uint32_t *vers)
> >     return true;
> > }
> >
> > +static int32_t get_ffa_ret_code(const struct arm_smccc_1_2_regs *resp)
> > +{
> > +    switch ( resp->a0 )
> > +    {
> > +    case FFA_ERROR:
> > +        if ( resp->a2 )
> > +            return resp->a2;
> > +        else
> > +            return FFA_RET_NOT_SUPPORTED;
> > +    case FFA_SUCCESS_32:
> > +    case FFA_SUCCESS_64:
> > +        return FFA_RET_OK;
> > +    default:
> > +        return FFA_RET_NOT_SUPPORTED;
> > +    }
> > +}
> > +
> > +static int32_t ffa_simple_call(uint32_t fid, register_t a1, register_t a2,
> > +                               register_t a3, register_t a4)
> > +{
> > +    const struct arm_smccc_1_2_regs arg = {
> > +        .a0 = fid,
> > +        .a1 = a1,
> > +        .a2 = a2,
> > +        .a3 = a3,
> > +        .a4 = a4,
> > +    };
> > +    struct arm_smccc_1_2_regs resp;
> > +
> > +    arm_smccc_1_2_smc(&arg, &resp);
> > +
> > +    return get_ffa_ret_code(&resp);
> > +}
> > +
> > +static int32_t ffa_features(uint32_t id)
> > +{
> > +    return ffa_simple_call(FFA_FEATURES, id, 0, 0, 0);
> > +}
> > +
> > +static bool check_mandatory_feature(uint32_t id)
> > +{
> > +    uint32_t ret = ffa_features(id);
> > +
> > +    if (ret)
> > +        printk(XENLOG_ERR "ffa: mandatory feature id %#x missing\n", id);
>
> It might be useful here to actually print the error code.
> Are we sure that all errors actually mean not supported ?

Yes, that's what the standard says.

>
> > +
> > +    return !ret;
> > +}
> > +
> > static uint16_t get_vm_id(const struct domain *d)
> > {
> >     /* +1 since 0 is reserved for the hypervisor in FF-A */
> > @@ -208,6 +258,66 @@ static void handle_version(struct cpu_user_regs *regs)
> >     set_regs(regs, vers, 0, 0, 0, 0, 0, 0, 0);
> > }
> >
> > +static void handle_msg_send_direct_req(struct cpu_user_regs *regs, 
> > uint32_t fid)
> > +{
> > +    struct arm_smccc_1_2_regs arg = { .a0 = fid, };
> > +    struct arm_smccc_1_2_regs resp = { };
> > +    struct domain *d = current->domain;
> > +    struct ffa_ctx *ctx = d->arch.tee;
> > +    uint32_t src_dst;
> > +    uint64_t mask;
> > +
> > +    if ( smccc_is_conv_64(fid) )
> > +        mask = GENMASK_ULL(63, 0);
> > +    else
> > +        mask = GENMASK_ULL(31, 0);
> > +
> > +    src_dst = get_user_reg(regs, 1);
> > +    if ( (src_dst >> 16) != get_vm_id(d) )
> > +    {
> > +        resp.a0 = FFA_ERROR;
> > +        resp.a2 = FFA_RET_INVALID_PARAMETERS;
> > +        goto out;
> > +    }
> > +
> > +    arg.a1 = src_dst;
> > +    arg.a2 = get_user_reg(regs, 2) & mask;
> > +    arg.a3 = get_user_reg(regs, 3) & mask;
> > +    arg.a4 = get_user_reg(regs, 4) & mask;
> > +    arg.a5 = get_user_reg(regs, 5) & mask;
> > +    arg.a6 = get_user_reg(regs, 6) & mask;
> > +    arg.a7 = get_user_reg(regs, 7) & mask;
> > +
> > +    while ( true )
> > +    {
> > +        arm_smccc_1_2_smc(&arg, &resp);
> > +
> > +        switch ( resp.a0 )
> > +        {
> > +        case FFA_INTERRUPT:
> > +            ctx->interrupted = true;
> > +            goto out;
> > +        case FFA_ERROR:
> > +        case FFA_SUCCESS_32:
> > +        case FFA_SUCCESS_64:
> > +        case FFA_MSG_SEND_DIRECT_RESP_32:
> > +        case FFA_MSG_SEND_DIRECT_RESP_64:
> > +            goto out;
> > +        default:
> > +            /* Bad fid, report back. */
> > +            memset(&arg, 0, sizeof(arg));
> > +            arg.a0 = FFA_ERROR;
> > +            arg.a1 = src_dst;
> > +            arg.a2 = FFA_RET_NOT_SUPPORTED;
> > +            continue;
>
> There is a potential infinite loop here and i do not understand
> why this needs to be done.
> Here if something is returning a value that you do not understand
> you send back an ERROR to it. I do not find in the spec where this
> is supposed to be done.
> Can you explain a bit here ?

This should normally not happen, but the SP/SPMC is responding with a
request that we don't know what to do with. The standard doesn't say
how to handle that as far as I understand. However, returning back to
the VM at this point with an error may leave the SP/SPMC in a strange
state. So I think it's better to report back to the SP/SPMC that the
request isn't understood and hopefully it can at least return back
with an error in a sane state.

I'll add something to the comment.

>
> > +        }
> > +    }
> > +
> > +out:
> > +    set_regs(regs, resp.a0, resp.a1 & mask, resp.a2 & mask, resp.a3 & mask,
> > +             resp.a4 & mask, resp.a5 & mask, resp.a6 & mask, resp.a7 & 
> > mask);
> > +}
> > +
> > static bool ffa_handle_call(struct cpu_user_regs *regs)
> > {
> >     uint32_t fid = get_user_reg(regs, 0);
> > @@ -225,6 +335,12 @@ static bool ffa_handle_call(struct cpu_user_regs *regs)
> >     case FFA_ID_GET:
> >         set_regs_success(regs, get_vm_id(d), 0);
> >         return true;
> > +    case FFA_MSG_SEND_DIRECT_REQ_32:
> > +#ifdef CONFIG_ARM_64
> > +    case FFA_MSG_SEND_DIRECT_REQ_64:
> > +#endif
> > +        handle_msg_send_direct_req(regs, fid);
> > +        return true;
> >
> >     default:
> >         gprintk(XENLOG_ERR, "ffa: unhandled fid 0x%x\n", fid);
> > @@ -310,6 +426,9 @@ static bool ffa_probe(void)
> >     printk(XENLOG_INFO "ARM FF-A Firmware version %u.%u\n",
> >            major_vers, minor_vers);
> >
> > +    if ( !check_mandatory_feature(FFA_MSG_SEND_DIRECT_REQ_32) )
> > +        return false;
>
> One could not need this feature and here this will make everything 
> unavailable instead.
> Why not just reporting back the unsupported error to clients using 
> unsupported interfaces ?

One could perhaps argue that this check should be moved to a later
patch in this series. Perhaps there's some future configuration that
might make sense without this feature, but for now, it doesn't make
sense to initialize without it.

Thanks,
Jens

>
> Cheers
> Bertrand
>
> > +
> >     ffa_version = vers;
> >
> >     return true;
> > --
> > 2.34.1
> >
>