Re: mmap in PV xen-4.0.1

[Eric Camachat <eric.camachat@gmail.com>]

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On Wed, Aug 10, 2011 at 2:12 AM, Wei Liu <liuw@liuw.name> wrote:
> 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.
>

I want share some memory between kernel and user applications, so I am
trying to use mmap instead of copy_{from,to} multiple times.

>>                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...

I am using linux-2.5.32.24 kernel.

>
>>        .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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