Re: [Qemu-devel] [PATCH v5 02/18] vfio: introduce vfio_get_vaddr()

[Paolo Bonzini]

On 26/01/2017 08:12, Peter Xu wrote:
> 
>     /*
>      * Here, we need to have the lock not only for vfio_get_vaddr(),
>      * but also needs to make sure that the vaddr will be valid for
>      * further operations.
>      *
>      * When we map new pages, we need the lock to make sure that vaddr
>      * is valid along the way we build up the IO page table (via
>      * vfio_dma_map()). Then, as long as the mapping is set up, we can
>      * unlock since those pages will be pinned in kernel (which makes
>      * sure that the RAM backend of vaddr will always be there, even
>      * if the memory object is destroyed and RAM released).
>      *
>      * For unmapping case, we don't really need the protection since
>      * the pages are in all cases locked in kernel, so we'll probably
>      * be safe even without the lock. However, it won't hurt we have
>      * the lock as well here.
>      */

Even simpler, just before the definition of vfio_get_vaddr:

/* Called with rcu_read_lock held.  */

and just before the vfio_dma_map call:

    /* vaddr is only valid until rcu_read_unlock().  But after
     * vfio_dma_map has set up the mapping the pages will be pinned
     * by the kernel.  This makes sure that the RAM backend of vaddr
     * will always be there, even if the memory object is destroyed
     * and its backing memory munmap-ed.
     */

I'm not sure that you can get rid of the lock for the unmapping case.
Better remove that part of the comment.

Thanks,

Paolo