Re: [Xen-devel] mmap in PV xen-4.0.1

[Wei Liu <liuw@xxxxxxxxx>]

On Tue, Aug 09, 2011 at 11:29:51PM -0700, Eric Camachat wrote:
> Hi,
> 
> I have a problem to map kernel memory to userspace via /dev/mem.
> The mmap() succeeded, but when I try to access it, the program will
> hang forever (until press ctrl-c to terminate it).
> 
> # memtest-user
> memtest_vma_open: virt 0x7fbc90085000, phys 0x3eee8000
> paddr = 0x3eee8000
>  mem = 0x7fbc90089000
>  map = 0x7fbc90085000
> map[0]= 4c4c4c4c
> map[1]= 4c4c4c4c
> *** Hang here, it cannot (finish) access the memory mapped via /dev/mem ***
> 
> My test source below, and it runs properly on HVM, VirtualBox, QEM and
> physical machines.
> What mistake I did?
> 
> My kernel module look like this:
> =================================================================================

[...snip...]

> memtest_ioctl(struct inode *inode, struct file *file, unsigned int cmd,
>               unsigned long arg)
> {
>        int ret = -ENOIOCTLCMD;
>        phys_addr_t *paddr;
>        unsigned long *vaddr;
>        uint32_t *size;
> 
>        switch(cmd) {
>        case MEMTEST_DMA_SIZE:
>                size = (uint32_t*)arg;
>                *size = _size;

Though your output shows that this assignment works, shouldn't this
kind of direct assignment across kernel space and user land be
avoided? It is bad practice to do direct assignment I think.

copy_{from,to}_user should do the job.

>                ret = 0;
>                break;
>        case MEMTEST_DMA_PADDR:
>                paddr = (phys_addr_t*)arg;
>                *paddr = _pbase;
>                ret = 0;
>                break;
>        case MEMTEST_DMA_VADDR:
>                vaddr = (unsigned long*)arg;
>                *vaddr = _vbase;
>                ret = 0;
>                break;
>        }
>        return ret;
> }
> 
>

[...snip...]
 
> static struct file_operations memtest_fops = {
>        .owner          = THIS_MODULE,
>        .llseek         = no_llseek,
>        .ioctl          = memtest_ioctl,

My kernel doesn't have field called 'ioctl' in file_operations.

So which kernel do you use? 2.6.18? I don't have old kernel at the
moment so I can't help you much...

>        .mmap           = memtest_mmap,
> };
> 
>

[...snip...]
 
> static void __exit memtest_exit(void)
> {
>        if (_vbase != 0)
>                free_page(_vbase);

I suppose you should use free_pages here, since you use
__get_free_pages when allocating.

>        unregister_chrdev(MEMTEST_MAJOR, MEMTEST_NAME);
> }
> 
> 
> MODULE_LICENSE("GPL");
> 
> module_init(memtest_init);
> module_exit(memtest_exit);
> =================================================================================
> 
> Here is my user program:
> 
> =================================================================================
>

[...snip...]
 
>       if (map)
>       {
>               printf("map[0]= %x\n", map[0]);
>               printf("map[1]= %x\n", map[1]);

This confuses me. You did write different values in _vbase[0],
_vbase[1]. But the output '4C4C4C4C' shows that the value is 'L'.

I just skimmed the output and the code. I don't run your code since I
don't have a suitible environment at the moment...

Wei.

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