Re: [PATCH 4/6] vhost_vdpa: support doorbell mapping via mmap

[Jason Wang <jasowang@redhat.com>]

On 2020/6/2 下午9:31, Michael S. Tsirkin wrote:
> On Tue, Jun 02, 2020 at 02:49:38PM +0800, Jason Wang wrote:
>> On 2020/6/2 下午12:56, Michael S. Tsirkin wrote:
>>> On Tue, Jun 02, 2020 at 03:22:49AM +0800, kbuild test robot wrote:
>>>> Hi Jason,
>>>>
>>>> I love your patch! Yet something to improve:
>>>>
>>>> [auto build test ERROR on vhost/linux-next]
>>>> [also build test ERROR on linus/master v5.7 next-20200529]
>>>> [if your patch is applied to the wrong git tree, please drop us a note to help
>>>> improve the system. BTW, we also suggest to use '--base' option to specify the
>>>> base tree in git format-patch, please seehttps://stackoverflow.com/a/37406982]
>>>>
>>>> url:https://github.com/0day-ci/linux/commits/Jason-Wang/vDPA-doorbell-mapping/20200531-070834
>>>> base:https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git  linux-next
>>>> config: m68k-randconfig-r011-20200601 (attached as .config)
>>>> compiler: m68k-linux-gcc (GCC) 9.3.0
>>>> reproduce (this is a W=1 build):
>>>>           wgethttps://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross  -O ~/bin/make.cross
>>>>           chmod +x ~/bin/make.cross
>>>>           # save the attached .config to linux build tree
>>>>           COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=m68k
>>>>
>>>> If you fix the issue, kindly add following tag as appropriate
>>>> Reported-by: kbuild test robot<lkp@intel.com>
>>>>
>>>> All errors (new ones prefixed by >>, old ones prefixed by <<):
>>>>
>>>> drivers/vhost/vdpa.c: In function 'vhost_vdpa_fault':
>>>>>> drivers/vhost/vdpa.c:754:22: error: implicit declaration of function 'pgprot_noncached' [-Werror=implicit-function-declaration]
>>>> 754 |  vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>>>> |                      ^~~~~~~~~~~~~~~~
>>>>>> drivers/vhost/vdpa.c:754:22: error: incompatible types when assigning to type 'pgprot_t' {aka 'struct <anonymous>'} from type 'int'
>>>> cc1: some warnings being treated as errors
>>>>
>>>> vim +/pgprot_noncached +754 drivers/vhost/vdpa.c
>>>>
>>>>      742	
>>>>      743	static vm_fault_t vhost_vdpa_fault(struct vm_fault *vmf)
>>>>      744	{
>>>>      745		struct vhost_vdpa *v = vmf->vma->vm_file->private_data;
>>>>      746		struct vdpa_device *vdpa = v->vdpa;
>>>>      747		const struct vdpa_config_ops *ops = vdpa->config;
>>>>      748		struct vdpa_notification_area notify;
>>>>      749		struct vm_area_struct *vma = vmf->vma;
>>>>      750		u16 index = vma->vm_pgoff;
>>>>      751	
>>>>      752		notify = ops->get_vq_notification(vdpa, index);
>>>>      753	
>>>>    > 754		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>>>>      755		if (remap_pfn_range(vma, vmf->address & PAGE_MASK,
>>>>      756				    notify.addr >> PAGE_SHIFT, PAGE_SIZE,
>>>>      757				    vma->vm_page_prot))
>>>>      758			return VM_FAULT_SIGBUS;
>>>>      759	
>>>>      760		return VM_FAULT_NOPAGE;
>>>>      761	}
>>>>      762	
>>> Yes well, all this remapping clearly has no chance to work
>>> on systems without CONFIG_MMU.
>>
>> It looks to me mmap can work according to Documentation/nommu-mmap.txt. But
>> I'm not sure it's worth to bother.
>>
>> Thanks
>
> Well
>
> int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>                  unsigned long pfn, unsigned long size, pgprot_t prot)
> {
>          if (addr != (pfn << PAGE_SHIFT))
>                  return -EINVAL;
>
>          vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
>          return 0;
> }
> EXPORT_SYMBOL(remap_pfn_range);
>
>
> So things aren't going to work if you have a fixed PFN
> which is the case of the hardware device.


Looking at the implementation of some drivers e.g mtd_char. If I read 
the code correctly, we can do this by providing get_unmapped_area method 
and use physical address directly.

But start form CONFIG_MMU should be fine.  Do you prefer making 
vhost_vdpa depends on CONFIG_MMU or just fail mmap when CONFIG_MMU is 
not configured?

Thanks


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