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

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Date:   Wed, 3 Jun 2020 14:37:51 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        rob.miller@...adcom.com, lingshan.zhu@...el.com,
        eperezma@...hat.com, lulu@...hat.com
Subject: Re: [PATCH 4/6] vhost_vdpa: support doorbell mapping via mmap


On 2020/6/3 下午2:34, Michael S. Tsirkin wrote:
> On Wed, Jun 03, 2020 at 12:18:44PM +0800, Jason Wang wrote:
>> 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@...el.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
> I'd just not specify the mmap callback at all.


Ok, will do.

Thanks


>