Re: R/W HG memory mappings with kvm?

[Avi Kivity <avi@redhat.com>]

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.

>>> In my case addr == vma->vm_start, and vma->vm_pgoff == 0, so pfn ==0.
>>>        
>> How did you set up that vma?  It should point to the first pfn of your
>> special memory area.
>>      
> The vma was created with a remap_pfn_range call from another driver.
> Because this call sets VM_PFNMAP and VM_IO any get_user_pages(_fast)
> calls will fail.
>
> In this case the host driver was actually just remapping host memory,
> so I replaced the remap_pfn_range call with a nopage/fault vm_op. This
> allows the get_user_pages_fast call to succeed, and the mapping now
> works as expected. This is sufficient for my work at the moment.
>
>    

Well if the fix is correct we need it too.

> 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.

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