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

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Date:   Tue, 2 Jun 2020 14:49:38 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        kbuild test robot <lkp@...el.com>
Cc:     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 2020/6/2 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


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