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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 02/12] xen/arm: ffa: per-VM FFA_VERSION negotiation state
Hi Bertrand,
On Mon, Dec 15, 2025 at 3:50 PM Bertrand Marquis
<bertrand.marquis@xxxxxxx> wrote:
>
> Track FF-A version negotiation per VM and enforce that no FF-A ABI
> (other than FFA_VERSION) is processed before a guest has selected a
> version.
>
> Each ffa_ctx gains a dedicated guest_vers_lock, a negotiated version
> (guest_vers) and a guest_vers_tmp:
> - guest_vers is the version negotiated or 0 if no version has been
> negotiated. This must be used with ACCESS_ONCE when reading it without
> the spinlock taken.
> - guest_vers_tmp stores the version currently requested by a VM.
>
> The version requested is the one actually negotiated once a call
> different from FFA_VERSION is done to allow several attempts and as
> requested by FF-A specification.
> We always return our implementation version FFA_MY_VERSION, even if the
> version requested was different, as requested by FF-A specification.
>
> Any call other than FFA_VERSION is rejected until a version has been
> requested.
>
> Update all places in the code where guest_vers is used to use
> ACCESS_ONCE.
>
> This prevents partially initialised contexts from using the mediator
> and complies with the FF-A 1.2 FFA_VERSION semantics.
>
> Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> ---
> Changes in v2:
> - add comment on top of guest_vers to instruct to always use ACCESS_ONCE
> to access it.
> Changes in v1:
> - remove the guest_vers_negotiated and use guest_vers = 0 as condition
> for a version being negotiated instead
> - introduce guest_vers_tmp to store a requested version until it is
> becoming the one negotiated.
> - remove not needed if negotiated condition.
> - use ACCESS_ONCE when reading guest_vers and use guest_vers == 0 as
> condition for a version being negotiated.
> - Update FF-A version handling comment in ffa_private.h
> ---
> xen/arch/arm/tee/ffa.c | 101 +++++++++++++++++++++++++-------
> xen/arch/arm/tee/ffa_msg.c | 2 +-
> xen/arch/arm/tee/ffa_partinfo.c | 4 +-
> xen/arch/arm/tee/ffa_private.h | 27 +++++++--
> xen/arch/arm/tee/ffa_shm.c | 3 +-
> 5 files changed, 106 insertions(+), 31 deletions(-)
Looks good.
Reviewed-by: Jens Wiklander <jens.wiklander@xxxxxxxxxx>
Cheers,
Jens
>
> diff --git a/xen/arch/arm/tee/ffa.c b/xen/arch/arm/tee/ffa.c
> index 2b4e24750d52..aadd6c21e7f2 100644
> --- a/xen/arch/arm/tee/ffa.c
> +++ b/xen/arch/arm/tee/ffa.c
> @@ -158,31 +158,38 @@ static bool ffa_abi_supported(uint32_t id)
> return !ffa_simple_call(FFA_FEATURES, id, 0, 0, 0);
> }
>
> -static void handle_version(struct cpu_user_regs *regs)
> +static bool ffa_negotiate_version(struct cpu_user_regs *regs)
> {
> struct domain *d = current->domain;
> struct ffa_ctx *ctx = d->arch.tee;
> - uint32_t vers = get_user_reg(regs, 1);
> - uint32_t old_vers;
> + uint32_t fid = get_user_reg(regs, 0);
> + uint32_t in_vers = get_user_reg(regs, 1);
> + uint32_t out_vers = FFA_MY_VERSION;
>
> - /*
> - * Guest will use the version it requested if it is our major and minor
> - * lower or equals to ours. If the minor is greater, our version will be
> - * used.
> - * In any case return our version to the caller.
> - */
> - if ( FFA_VERSION_MAJOR(vers) == FFA_MY_VERSION_MAJOR )
> + spin_lock(&ctx->guest_vers_lock);
> +
> + /* If negotiation already published, continue without handling. */
> + if ( ACCESS_ONCE(ctx->guest_vers) )
> + goto out_continue;
> +
> + if ( fid != FFA_VERSION )
> {
> - spin_lock(&ctx->lock);
> - old_vers = ctx->guest_vers;
> + if ( !ctx->guest_vers_tmp )
> + {
> + out_vers = 0;
> + goto out_handled;
> + }
>
> - if ( FFA_VERSION_MINOR(vers) > FFA_MY_VERSION_MINOR )
> - ctx->guest_vers = FFA_MY_VERSION;
> - else
> - ctx->guest_vers = vers;
> - spin_unlock(&ctx->lock);
> + /*
> + * A successful FFA_VERSION call does not freeze negotiation. Guests
> + * are allowed to issue multiple FFA_VERSION attempts (e.g. probing
> + * several minor versions). Negotiation becomes final only when a
> + * non-VERSION ABI is invoked, as required by the FF-A specification.
> + * Finalize negotiation: publish guest_vers once, then never change.
> + */
> + ACCESS_ONCE(ctx->guest_vers) = ctx->guest_vers_tmp;
>
> - if ( IS_ENABLED(CONFIG_FFA_VM_TO_VM) && !old_vers )
> + if ( IS_ENABLED(CONFIG_FFA_VM_TO_VM) )
> {
> /* One more VM with FF-A support available */
> inc_ffa_vm_count();
> @@ -190,8 +197,48 @@ static void handle_version(struct cpu_user_regs *regs)
> list_add_tail(&ctx->ctx_list, &ffa_ctx_head);
> write_unlock(&ffa_ctx_list_rwlock);
> }
> +
> + goto out_continue;
> }
> - ffa_set_regs(regs, FFA_MY_VERSION, 0, 0, 0, 0, 0, 0, 0);
> +
> + /*
> + * guest_vers_tmp stores the version selected by the guest (lower minor
> may
> + * require reduced data structures). However, the value returned to the
> + * guest via FFA_VERSION is always FFA_MY_VERSION, the implementation
> + * version, as required by FF-A. The two values intentionally differ.
> + */
> +
> + /*
> + * Return our highest implementation version on request different than
> our
> + * major and mark negotiated version as our implementation version.
> + */
> + if ( FFA_VERSION_MAJOR(in_vers) != FFA_MY_VERSION_MAJOR )
> + {
> + ctx->guest_vers_tmp = FFA_MY_VERSION;
> + goto out_handled;
> + }
> +
> + /*
> + * Use our minor version if a greater minor was requested or the
> requested
> + * minor if it is lower than ours was requested.
> + */
> + if ( FFA_VERSION_MINOR(in_vers) > FFA_MY_VERSION_MINOR )
> + ctx->guest_vers_tmp = FFA_MY_VERSION;
> + else
> + ctx->guest_vers_tmp = in_vers;
> +
> +out_handled:
> + spin_unlock(&ctx->guest_vers_lock);
> + if ( out_vers )
> + ffa_set_regs(regs, out_vers, 0, 0, 0, 0, 0, 0, 0);
> + else
> + ffa_set_regs_error(regs, FFA_RET_NOT_SUPPORTED);
> + return true;
> +
> +out_continue:
> + spin_unlock(&ctx->guest_vers_lock);
> +
> + return false;
> }
>
> static void handle_features(struct cpu_user_regs *regs)
> @@ -274,10 +321,17 @@ static bool ffa_handle_call(struct cpu_user_regs *regs)
> if ( !ctx )
> return false;
>
> + /* A version must be negotiated first */
> + if ( !ACCESS_ONCE(ctx->guest_vers) )
> + {
> + if ( ffa_negotiate_version(regs) )
> + return true;
> + }
> +
> switch ( fid )
> {
> case FFA_VERSION:
> - handle_version(regs);
> + ffa_set_regs(regs, FFA_MY_VERSION, 0, 0, 0, 0, 0, 0, 0);
> return true;
> case FFA_ID_GET:
> ffa_set_regs_success(regs, ffa_get_vm_id(d), 0);
> @@ -371,6 +425,11 @@ static int ffa_domain_init(struct domain *d)
> d->arch.tee = ctx;
> ctx->teardown_d = d;
> INIT_LIST_HEAD(&ctx->shm_list);
> + spin_lock_init(&ctx->lock);
> + spin_lock_init(&ctx->guest_vers_lock);
> + ctx->guest_vers = 0;
> + ctx->guest_vers_tmp = 0;
> + INIT_LIST_HEAD(&ctx->ctx_list);
>
> ctx->ffa_id = ffa_get_vm_id(d);
> ctx->num_vcpus = d->max_vcpus;
> @@ -452,7 +511,7 @@ static int ffa_domain_teardown(struct domain *d)
> if ( !ctx )
> return 0;
>
> - if ( IS_ENABLED(CONFIG_FFA_VM_TO_VM) && ctx->guest_vers )
> + if ( IS_ENABLED(CONFIG_FFA_VM_TO_VM) && ACCESS_ONCE(ctx->guest_vers) )
> {
> dec_ffa_vm_count();
> write_lock(&ffa_ctx_list_rwlock);
> diff --git a/xen/arch/arm/tee/ffa_msg.c b/xen/arch/arm/tee/ffa_msg.c
> index c20c5bec0f76..2c2ebc9c5cd6 100644
> --- a/xen/arch/arm/tee/ffa_msg.c
> +++ b/xen/arch/arm/tee/ffa_msg.c
> @@ -113,7 +113,7 @@ static int32_t ffa_msg_send2_vm(uint16_t dst_id, const
> void *src_buf,
> }
>
> dst_ctx = dst_d->arch.tee;
> - if ( !dst_ctx->guest_vers )
> + if ( !ACCESS_ONCE(dst_ctx->guest_vers) )
> {
> ret = FFA_RET_INVALID_PARAMETERS;
> goto out_unlock;
> diff --git a/xen/arch/arm/tee/ffa_partinfo.c b/xen/arch/arm/tee/ffa_partinfo.c
> index fa56b1587e3b..ec5a53ed1cab 100644
> --- a/xen/arch/arm/tee/ffa_partinfo.c
> +++ b/xen/arch/arm/tee/ffa_partinfo.c
> @@ -238,7 +238,7 @@ void ffa_handle_partition_info_get(struct cpu_user_regs
> *regs)
> * use the v1.0 structure size in the destination buffer.
> * Otherwise use the size of the highest version we support, here 1.1.
> */
> - if ( ctx->guest_vers == FFA_VERSION_1_0 )
> + if ( ACCESS_ONCE(ctx->guest_vers) == FFA_VERSION_1_0 )
> dst_size = sizeof(struct ffa_partition_info_1_0);
> else
> dst_size = sizeof(struct ffa_partition_info_1_1);
> @@ -250,7 +250,7 @@ void ffa_handle_partition_info_get(struct cpu_user_regs
> *regs)
> * FF-A v1.0 has w5 MBZ while v1.1 allows
> * FFA_PARTITION_INFO_GET_COUNT_FLAG to be non-zero.
> */
> - if ( ctx->guest_vers == FFA_VERSION_1_0 ||
> + if ( ACCESS_ONCE(ctx->guest_vers) == FFA_VERSION_1_0 ||
> flags != FFA_PARTITION_INFO_GET_COUNT_FLAG )
> {
> ret = FFA_RET_INVALID_PARAMETERS;
> diff --git a/xen/arch/arm/tee/ffa_private.h b/xen/arch/arm/tee/ffa_private.h
> index 8d01da0009d3..8ef214344711 100644
> --- a/xen/arch/arm/tee/ffa_private.h
> +++ b/xen/arch/arm/tee/ffa_private.h
> @@ -355,12 +355,6 @@ struct ffa_ctx {
> * Global data accessed with lock locked.
> */
> spinlock_t lock;
> - /*
> - * FF-A version negotiated by the guest, only modifications to
> - * this field are done with the lock held as this is expected to
> - * be done once at init by a guest.
> - */
> - uint32_t guest_vers;
> /* Number of 4kB pages in each of rx/rx_pg and tx/tx_pg */
> unsigned int page_count;
> /* Number of allocated shared memory object */
> @@ -368,6 +362,27 @@ struct ffa_ctx {
> /* Used shared memory objects, struct ffa_shm_mem */
> struct list_head shm_list;
>
> + /*
> + * FF-A version handling
> + * guest_vers is the single published negotiated version. It is 0 until
> + * negotiation completes, after which it is set once and never changes.
> + * Negotiation uses guest_vers_tmp under guest_vers_lock; when a
> + * non-VERSION ABI is invoked, Xen finalizes negotiation by publishing
> + * guest_vers using ACCESS_ONCE() store.
> + * Readers use ACCESS_ONCE(guest_vers) != 0 to detect availability and
> + * can consume guest_vers without barriers because it never changes once
> + * published.
> + */
> + spinlock_t guest_vers_lock;
> + /*
> + * Published negotiated version. Zero means "not negotiated yet".
> + * Once non-zero, it never changes.
> + * Must always be accessed using ACCESS_ONCE().
> + */
> + uint32_t guest_vers;
> + /* Temporary version used during negotiation under guest_vers_lock */
> + uint32_t guest_vers_tmp;
> +
> /*
> * Rx buffer, accessed with rx_lock locked.
> * rx_is_free is used to serialize access.
> diff --git a/xen/arch/arm/tee/ffa_shm.c b/xen/arch/arm/tee/ffa_shm.c
> index d628c1b70609..dad3da192247 100644
> --- a/xen/arch/arm/tee/ffa_shm.c
> +++ b/xen/arch/arm/tee/ffa_shm.c
> @@ -495,7 +495,8 @@ void ffa_handle_mem_share(struct cpu_user_regs *regs)
> if ( frag_len > ctx->page_count * FFA_PAGE_SIZE )
> goto out_unlock;
>
> - ret = read_mem_transaction(ctx->guest_vers, ctx->tx, frag_len, &trans);
> + ret = read_mem_transaction(ACCESS_ONCE(ctx->guest_vers), ctx->tx,
> + frag_len, &trans);
> if ( ret )
> goto out_unlock;
>
> --
> 2.51.2
>
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