Re: R/W HG memory mappings with kvm?

[Avi Kivity <avi@redhat.com>]

On 08/27/2009 05:34 AM, Stephen Donnelly wrote:
> On Mon, Aug 24, 2009 at 4:55 PM, Avi Kivity<avi@redhat.com>  wrote:
>    
>> On 08/24/2009 12:59 AM, Stephen Donnelly wrote:
>>      
>>> On Thu, Aug 20, 2009 at 12:14 AM, Avi Kivity<avi@redhat.com>    wrote:
>>>        
>>>> On 08/13/2009 07:07 AM, Stephen Donnelly wrote:
>>>>          
>>>>> npages = get_user_pages_fast(addr, 1, 1, page); returns -EFAULT,
>>>>> presumably because (vma->vm_flags&      (VM_IO | VM_PFNMAP)).
>>>>>
>>>>> It takes then unlikely branch, and checks the vma, but I don't
>>>>> understand what it is doing here: pfn = ((addr - vma->vm_start)>>
>>>>> PAGE_SHIFT) + vma->vm_pgoff;
>>>>>            
>>>> It's calculating the pfn according to pfnmap rules.
>>>>          
>>>   From what I understand this will only work when remapping 'main
>>> memory', e.g. where the pgoff is equal to the physical page offset?
>>> VMAs that remap IO memory will usually set pgoff to 0 for the start of
>>> the mapping.
>>>        
>> If so, how do they calculate the pfn when mapping pages?  kvm needs to be
>> able to do the same thing.
>>      
> If the vma->vm_file is /dev/mem, then the pg_off will map to physical
> addresses directly (at least on x86), and the calculation works. If
> the vma is remapping io memory from a driver, then vma->vm_file will
> point to the device node for that driver. Perhaps we can do a check
> for this at least?
>    

We can't duplicate mm/ in kvm.  However, mm/memory.c says:


  * The way we recognize COWed pages within VM_PFNMAP mappings is 
through the
  * rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit
  * set, and the vm_pgoff will point to the first PFN mapped: thus every 
special
  * mapping will always honor the rule
  *
  *      pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> 
PAGE_SHIFT)
  *
  * And for normal mappings this is false.

So it seems the kvm calculation is right and you should set vm_pgoff in 
your driver.

>
>
>>> I'm still not sure how genuine IO memory (mapped from a driver to
>>> userspace with remap_pfn_range or io_remap_page_range) could be mapped
>>> into kvm though.
>>>        
>> If it can be mapped to userspace, it can be mapped to kvm.  We just need to
>> synchronize the rules.
>>      
> We can definitely map it into userspace. The problem seems to be how
> the kvm kernel module translates the guest pfn back to a host physical
> address.
>
> Is there a kernel equivalent of mmap?
>    

do_mmap(), but don't use it.  Use mmap() from userspace like everyone else.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.