On Tue, Sep 22, 2026 at 09:58:53AM -0600, Mathieu Poirier wrote: > On Wed, Sep 16, 2026 at 11:10:51PM +0200, Francesco Valla wrote: > > Depending on the driver originating them, data buffers used for virtio > > communication can either: > > > > - already be allocated from the coherent memory area that is > > accessible by the remote processor; this is the case of rpmsg > > and the rproc flavor of virtio-console; > > - be allocated from generic kmem, and thus not accessible directly by > > the remote processor. > > > > Exploiting the map operations, which are used by the virtio framework > > when VIRTIO_F_ACCESS_PLATFORM is part of a vdev's feature flags, add > > bounce buffering for the second case: when the map() callback is called > > for a buffer, one or more pages of coherent memory are allocated and > > data is copied to them, then they are exposed to the remote processor; > > the data is then bounced back on unmap(). > > > > The first case is not impacted, since buffers already suitable for > > remote transmission are passed through. > > > > With the bounce buffering in place, any kind of virtio device can be > > supported through the remoteproc-virtio transport, at least from a > > data exchange standpoint. > > Is this _necessary_ for the imx93 platform you are implementing feature for? >
If I don't want to fundamentally change how the remoteproc integration works (i.e.: using buffers only from a pre-shared area), yes. While in my test environment the Cortex-M33 serving as remoteproc is able to access the whole RAM space, that is not always the case. This is even more important for mixed-criticality-ready processors (say: i.MX95), where access to different memory areas si defined even before the various application cores are started. For the sake of completeness, there *might be* a completely different approach that would work: modify every virtio driver to make sure they allocate buffers from the pre-shared memory area (that is, the coherent memory pool). That approach would even avoid the bounce buffering and allow for zero-copy transmission. I even started working on that, defining virtio helpers to do the allocations instead of using raw kmallocs, but then I dropped the idea because of the magnitude of the changes required. > > > > Signed-off-by: Francesco Valla <[email protected]> > > --- > > drivers/remoteproc/remoteproc_virtio.c | 182 > > +++++++++++++++++++++++++++++++-- > > include/linux/remoteproc.h | 14 +++ > > 2 files changed, 190 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/remoteproc/remoteproc_virtio.c > > b/drivers/remoteproc/remoteproc_virtio.c > > index cfd66d9d1c9e..d21b3b8044df 100644 > > --- a/drivers/remoteproc/remoteproc_virtio.c > > +++ b/drivers/remoteproc/remoteproc_virtio.c > > @@ -241,7 +241,14 @@ static void rproc_virtio_reset(struct virtio_device > > *vdev) > > dev_dbg(&vdev->dev, "reset !\n"); > > } > > > > -/* provide the vdev features as retrieved from the firmware */ > > +/* Provide the vdev features as retrieved from the firmware, plus the > > following > > + * additional ones: > > + * - VIRTIO_F_VERSION_1 that is required by some non-rpmsg virtio devices > > + * - VIRTIO_F_ACCESS_PLATFORM to force usage of the map operations > > + */ > > +#define RPROC_VIRTIO_STATIC_FEATURES \ > > + ((1ULL << VIRTIO_F_VERSION_1) | (1ULL << VIRTIO_F_ACCESS_PLATFORM)) > > + > > static u64 rproc_virtio_get_features(struct virtio_device *vdev) > > { > > struct rproc_vdev *rvdev = vdev_to_rvdev(vdev); > > @@ -249,7 +256,7 @@ static u64 rproc_virtio_get_features(struct > > virtio_device *vdev) > > > > rsc = (void *)rvdev->rproc->table_ptr + rvdev->rsc_offset; > > > > - return rsc->dfeatures | (1ULL << VIRTIO_F_VERSION_1); > > + return rsc->dfeatures | RPROC_VIRTIO_STATIC_FEATURES; > > } > > > > static void rproc_transport_features(struct virtio_device *vdev) > > @@ -275,16 +282,16 @@ static int rproc_virtio_finalize_features(struct > > virtio_device *vdev) > > /* Give virtio_rproc a chance to accept features. */ > > rproc_transport_features(vdev); > > > > - /* Make sure we don't have any features > 32 bits except > > VIRTIO_F_VERSION_1 */ > > + /* Make sure we don't have any features > 32 bits */ > > if (WARN_ON_ONCE((u32)vdev->features != > > - (vdev->features & ~(1ULL << VIRTIO_F_VERSION_1)))) > > + (vdev->features & ~RPROC_VIRTIO_STATIC_FEATURES))) > > return -1; > > > > /* > > * Remember the finalized features of our vdev, and provide it > > * to the remote processor once it is powered on. > > */ > > - rsc->gfeatures = vdev->features & ~(1ULL << VIRTIO_F_VERSION_1); > > + rsc->gfeatures = vdev->features & ~RPROC_VIRTIO_STATIC_FEATURES; > > > > return 0; > > } > > @@ -337,6 +344,151 @@ static const struct virtio_config_ops > > rproc_virtio_config_ops = { > > .set = rproc_virtio_set, > > }; > > > > +static inline unsigned int rproc_virtio_bounce_slot(struct device *dma_dev, > > + dma_addr_t dma_handle) > > +{ > > + const dma_addr_t dma_base = dma_dev_coherent_base(dma_dev); > > + > > + return (dma_handle - dma_base) >> PAGE_SHIFT; > > +} > > + > > +static dma_addr_t rproc_virtio_map_page(union virtio_map map, struct page > > *page, > > + unsigned long offset, size_t size, > > + enum dma_data_direction dir, > > + unsigned long attrs) > > +{ > > + struct device *dev = map.dma_dev; > > + struct rproc_vdev *rvdev = dev_get_drvdata(dev); > > + dma_addr_t dma_base = dma_dev_coherent_base(dev); > > + size_t dma_size = dma_dev_coherent_size(dev); > > + phys_addr_t paddr = page_to_phys(page) + offset; > > + void *vaddr = page_to_virt(page) + offset; > > + struct rproc_map_record *record; > > + dma_addr_t map_handle; > > + void *bounce; > > + > > + // No need to allocate a bounce buffer if the memory to map is already > > + // part of the device's coherent pool. > > + if (paddr >= dma_base && paddr < (dma_base + dma_size)) { > > + // The allocation details will be recorded also in this case, > > + // indicating that no bounce buffer was allocated. > > + map_handle = (dma_addr_t)paddr; > > + bounce = NULL; > > + } else { > > + // Allocate bounce buffer from device coherent memory > > + bounce = dma_alloc_coherent(dev, size, &map_handle, GFP_KERNEL > > | __GFP_ZERO); > > + if (!bounce) > > + return DMA_MAPPING_ERROR; > > + > > + // Copy data to bounce buffer > > + memcpy(bounce, vaddr, size); > > + } > > + > > + // Save bounce details > > + record = &rvdev->map_records[rproc_virtio_bounce_slot(dev, map_handle)]; > > + > > + record->original = vaddr; > > + record->size = size; > > + record->bounce = bounce; > > + > > + return map_handle; > > +} > > + > > +static void rproc_virtio_unmap_page(union virtio_map map, dma_addr_t > > map_handle, > > + size_t size, enum dma_data_direction dir, > > + unsigned long attrs) > > +{ > > + struct device *dev = map.dma_dev; > > + struct rproc_vdev *rvdev = dev_get_drvdata(dev); > > + unsigned int slot = rproc_virtio_bounce_slot(dev, map_handle); > > + struct rproc_map_record *record = &rvdev->map_records[slot]; > > + > > + WARN_ON(size != record->size); > > + > > + // If a bounce buffer was used, copy data back to original one > > + if (record->bounce) { > > + memcpy(record->original, record->bounce, record->size); > > + > > + dma_free_coherent(dev, record->size, record->bounce, > > map_handle); > > + } > > + > > + record->original = NULL; > > + record->size = 0; > > + record->bounce = NULL; > > +} > > + > > +static void rproc_virtio_sync_single_for_cpu(union virtio_map map, > > + dma_addr_t map_handle, > > + size_t size, > > + enum dma_data_direction dir) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + dma_sync_single_range_for_cpu(dev, (map_handle & PAGE_MASK), > > + offset_in_page(map_handle), size, dir); > > +} > > + > > +static void rproc_virtio_sync_single_for_device(union virtio_map map, > > + dma_addr_t map_handle, > > + size_t size, > > + enum dma_data_direction dir) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + dma_sync_single_range_for_device(dev, (map_handle & PAGE_MASK), > > + offset_in_page(map_handle), size, dir); > > +} > > + > > +static void *rproc_virtio_alloc(union virtio_map map, size_t size, > > + dma_addr_t *map_handle, gfp_t gfp) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + return dma_alloc_coherent(dev, size, map_handle, gfp); > > +} > > + > > +static void rproc_virtio_free(union virtio_map map, size_t size, void > > *vaddr, > > + dma_addr_t map_handle, unsigned long attrs) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + dma_free_coherent(dev, size, vaddr, map_handle); > > +} > > + > > +static bool rproc_virtio_need_sync(union virtio_map map, dma_addr_t > > map_handle) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + return dma_need_sync(dev, map_handle); > > +} > > + > > +static int rproc_virtio_mapping_error(union virtio_map map, dma_addr_t > > map_handle) > > +{ > > + if (unlikely(map_handle == DMA_MAPPING_ERROR)) > > + return -ENOMEM; > > + > > + return 0; > > +} > > + > > +static inline size_t rproc_virtio_max_mapping_size(union virtio_map map) > > +{ > > + struct device *dev = map.dma_dev; > > + > > + return dma_dev_coherent_size(dev); > > +} > > + > > +static const struct virtio_map_ops rproc_virtio_map_ops = { > > + .map_page = rproc_virtio_map_page, > > + .unmap_page = rproc_virtio_unmap_page, > > + .sync_single_for_cpu = rproc_virtio_sync_single_for_cpu, > > + .sync_single_for_device = rproc_virtio_sync_single_for_device, > > + .alloc = rproc_virtio_alloc, > > + .free = rproc_virtio_free, > > + .need_sync = rproc_virtio_need_sync, > > + .mapping_error = rproc_virtio_mapping_error, > > + .max_mapping_size = rproc_virtio_max_mapping_size, > > +}; > > + > > /* > > * This function is called whenever vdev is released, and is responsible > > * to decrement the remote processor's refcount which was taken when vdev > > was > > @@ -355,6 +507,8 @@ static void rproc_virtio_dev_release(struct device *dev) > > of_reserved_mem_device_release(&rvdev->pdev->dev); > > dma_release_coherent_memory(&rvdev->pdev->dev); > > > > + kvfree(rvdev->map_records); > > + > > put_device(&rvdev->pdev->dev); > > } > > > > @@ -429,13 +583,29 @@ static int rproc_add_virtio_dev(struct rproc_vdev > > *rvdev, int id) > > of_reserved_mem_device_init_by_idx(dev, np, 0); > > } > > > > + /* Allocate one tracking record for each page of the device reserved > > + * memory. Contiguous memory is not required for this array, which can > > + * also be quite big (depending on the size of the coherent memory), so > > + * let's use vmalloc for this allocation. > > + */ > > + rvdev->map_records = kvcalloc(dma_dev_coherent_size(dev) >> PAGE_SHIFT, > > + sizeof(*rvdev->map_records), > > + GFP_KERNEL); > > + if (!rvdev->map_records) { > > + dev_err(dev, "failed to allocate memory for map records\n"); > > + return -ENOMEM; > > + } > > + > > /* Allocate virtio device */ > > vdev = kzalloc_obj(*vdev); > > - if (!vdev) > > + if (!vdev) { > > + kvfree(rvdev->map_records); > > return -ENOMEM; > > + } > > > > vdev->id.device = id; > > vdev->config = &rproc_virtio_config_ops; > > + vdev->map = &rproc_virtio_map_ops; > > vdev->dev.parent = dev; > > vdev->dev.release = rproc_virtio_dev_release; > > > > diff --git a/include/linux/remoteproc.h b/include/linux/remoteproc.h > > index c3ba51fe9e54..2ff48b505ac0 100644 > > --- a/include/linux/remoteproc.h > > +++ b/include/linux/remoteproc.h > > @@ -339,10 +339,23 @@ struct rproc_vring { > > struct virtqueue *vq; > > }; > > > > +/** > > + * struct rproc_map_record - remoteproc map record > > + * @original: original virtual address > > + * @num: allocation size > > + * @bounce: bounce buffer virtual address (NULL if not used) > > + */ > > +struct rproc_map_record { > > + void *original; > > + size_t size; > > + void *bounce; > > +}; > > + > > /** > > * struct rproc_vdev - remoteproc state for a supported virtio device > > * @subdev: handle for registering the vdev as a rproc subdevice > > * @pdev: remoteproc virtio platform device > > + * @map_records: array of map records > > * @id: virtio device id (as in virtio_ids.h) > > * @node: list node > > * @rproc: the rproc handle > > @@ -358,6 +371,7 @@ struct rproc_vdev { > > unsigned int id; > > struct list_head node; > > struct rproc *rproc; > > + struct rproc_map_record *map_records; > > u32 rsc_offset; > > u32 index; > > unsigned int num_vrings; > > > > -- > > 2.55.0 > > Regards, Francesco
