kernel - userspace shared memory

[Barry Song]

2011/6/7 Zoltan Devai <zdevai at gmail.com>:
> Hi all,
>
> We are having a kernel driver and a userspace application that
> communicates via shared memory. The problem is, that when the userspace
> application modifies some data in the shared memory, this modification
> is not "visible" when the kernel driver takes a look at that memory
> location (specifically this code is in a kernel timer function).
> Actually, the modification is sometimes seen, sometimes not,
> which suggests that we are having some kind of cache coherency issue.
> The strange thing is, that AFAIK we have disabled caching on the pages
> where the shared memory resides.
>
> Below is the code that we use to create the shared memory area.
>
> 1. Do you see any problems with it?
> 2. As it can be seen, we are disabling caching for the user space
>   mapping, maybe we need to explicitly disable it for the kernel
>   mapping too? If yes, how to access the PTEs that are doing the kernel
>   logical mappings and disable caching?
> 3. Any hints on how to debug the source of such an error?

what's your ARM core? is it VIPT or VIVT? and do you have DMA
operation? All those result in difference answers. If VIVT, it will be
easy to have cache aliasing issue.

>
> Any help is appreciated.
>
> Best regards,
> Istvan Devai
> (actually sent by Zoltan Devai, on his behalf)
> ----------------------------------------
>
> Kernel initialization code (error handling is omitted for clarity, also
> the code that rounds the memory to the nearest page boundary):
>
> kmalloc_ptr = kmalloc((SHAREDMEM_PAGE_NUM) * PAGE_SIZE, GFP_KERNEL);
> for (i = 0; i < SHAREDMEM_PAGE_NUM * PAGE_SIZE; i+= PAGE_SIZE) {
>            SetPageReserved(virt_to_page(((unsigned long)kmalloc_ptr) + i));
>    }
>
> Kernel mmap code:
>
> vma->vm_flags |= VM_RESERVED;
> vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>
> /* map the whole physically contiguous area in one piece */
> remap_pfn_range(vma,
>                           vma->vm_start,
>                           virt_to_phys((void *)kmalloc_ptr) >> PAGE_SHIFT,
>                           length,
>                           vma->vm_page_prot)) < 0)
>
> Userspace mmap code:
>
> mmap(0, len, PROT_READ|PROT_WRITE, MAP_SHARED| MAP_LOCKED, fd,
> MMAP_OFFSET_SHAREDMEM)
>
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