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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 2 Jun 2020 09:31:16 -0400

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<[email-masked]>
> > > 
> > > 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.