Associating Kernel Memory With User Mappings

Some device drivers might need to allocate kernel memory that is made accessible to user programs by using mmap(2). Examples of this are setting up shared memory for communication between two applications or between driver and application.

In general, the steps for exporting kernel memory to user applications are:

1. Allocate kernel memory using ddi_umem_alloc(9F).

2. Export the memory using devmap_umem_setup(9F).

3. Free the memory using ddi_umem_free(9F) when no longer needed.

Allocating Kernel Memory for User Access

ddi_umem_alloc(9F) is provided to allocate kernel memory that is exported to applications. The syntax for ddi_umem_alloc() is:

void *ddi_umem_alloc(size_t size, int flag, ddi_umem_cookie_t 
*cookiep);            

ddi_umem_alloc(9F) allocates page-aligned kernel memory and returns a pointer to the allocated memory. The initial contents of the memory is zero-filled. The number of bytes allocated is a multiple of the system page size (roundup of size). The allocated memory can be used in the kernel and can be exported to applications. cookiep is a pointer to the kernel memory cookie that describes the kernel memory being allocated. It is used in devmap_umem_setup(9F) when the driver exports the kernel memory to a user application.

The flag argument indicates whether ddi_umem_alloc(9F) will block or return immediately, and whether the allocated kernel memory is pageable. Table 12-1 lists the values for flag.

Table 12-1 ddi_umem_alloc(9F) flag Values

Example 12-2 shows how to allocate kernel memory for application access. The driver exports one page of kernel memory, which is used by multiple applications as a shared memory area. The memory is allocated in segmap(9E) when an application maps the shared page the first time. An additional page is allocated if the driver has to support multiple application data models (for example a 64-bit driver exporting memory to 64-bit and 32-bit applications). 64-bit applications share the first page, and 32-bit applications share the second page.

Example 12-2 ddi_umem_alloc(9F) Routine

static int
xxsegmap(dev_t dev, off_t off, struct as *asp, caddr_t *addrp, off_t len,
    unsigned int prot, unsigned int maxprot, unsigned int flags, 
    cred_t *credp)
{
        int error;
        minor_t instance = getminor(dev);
        struct xxstate *xsp = ddi_get_soft_state(statep, instance);

        size_t mem_size;
            /* 64-bit driver supports 64-bit and 32-bit applications */
        switch (ddi_mmap_get_model()) {
                case DDI_MODEL_LP64:
                     mem_size = ptob(2);
                     break;
                case DDI_MODEL_ILP32:
                     mem_size = ptob(1);
                     break;
        }
       
        mutex_enter(&xsp->mu);
        if (xsp->umem == NULL) {

                /* allocate the shared area as kernel pageable memory */
                xsp->umem = ddi_umem_alloc(mem_size,
                    DDI_UMEM_SLEEP | DDI_UMEM_PAGEABLE, &xsp->ucookie);
        }
        mutex_exit(&xsp->mu);
        /* Set up the user mapping */
        error = devmap_setup(dev, (offset_t)off, asp, addrp, len,
            prot, maxprot, flags, credp);
    
        return (error);
}

Exporting Kernel Memory to Applications

devmap_umem_setup(9F) is provided to export kernel memory to user applications. devmap_umem_setup() must be called from the driver's devmap(9E) entry point. The syntax for devmap_umem_setup() is as follows:

int devmap_umem_setup(devmap_cookie_t handle, dev_info_t *dip,
    struct devmap_callback_ctl *callbackops, ddi_umem_cookie_t cookie,
    offset_t koff, size_t len, uint_t maxprot, uint_t flags,
    ddi_device_acc_attr_t *accattrp);

handle is a device-mapping handle that the system uses to identify the mapping. It is passed in by the devmap(9E) entry point. dip is a pointer to the device's dev_info structure. callbackops allows the driver to be notified of user events on the mapping. Most drivers set callbackops to NULL when kernel memory is exported.

koff and len specify a range within the kernel memory allocated by ddi_umem_alloc(9F). This range will be made accessible to the user's application mapping at the offset passed in by the devmap(9E) entry point. Usually the driver will pass the devmap(9E) offset directly to devmap_umem_setup(9F). The return address of mmap(2) will then map to the kernel address returned by ddi_umem_alloc(9F). koff and len must be page-aligned.

maxprot enables the driver to specify different protections for different regions within the exported kernel memory. For example, one region might not allow write access by only setting PROT_READ and PROT_USER.

Example 12-3 shows how to export kernel memory to an application. The driver first checks if the requested mapping falls within the allocated kernel memory region. If a 64-bit driver receives a mapping request from a 32-bit application, the request is redirected to the second page of the kernel memory area. This ensures that only applications compiled to the same data model will share the same page.

Example 12-3 devmap_umem_setup(9F) Routine

static int
xxdevmap(dev_t dev, devmap_cookie_t handle, offset_t off, size_t len,
    size_t *maplen, uint_t model)
{
        struct xxstate *xsp;
        int    error;

        /* round up len to a multiple of a page size */
        len = ptob(btopr(len));
        /* check if the requested range is ok */
        if (off + len > ptob(1))
                return (ENXIO);
        xsp = ddi_get_soft_state(statep, getminor(dev));
        if (xsp == NULL)
                return (ENXIO);

        if (ddi_model_convert_from(model) == DDI_MODEL_ILP32)
                /* request from 32-bit application. Skip first page */
                off += ptob(1);

        /* export the memory to the application */
        error = devmap_umem_setup(handle, xsp->dip, NULL, xsp->ucookie,
            off, len, PROT_ALL, DEVMAP_DEFAULTS, NULL);
        *maplen = len;
        return (error);
}