Re: R/W HG memory mappings with kvm?

[Stephen Donnelly <sfdonnelly@gmail.com>]

On Wed, Aug 26, 2009 at 10:22 PM, Avi Kivity<avi@redhat.com> wrote:
> On 08/24/2009 07:55 AM, Avi Kivity 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.
>
> Maybe the simplest thing is to call vma->vm_ops->fault here.  Marcelo/Chris?
>  Context is improving gfn_to_pfn() on the mmio path.

If the mapping is made using remap_pfn_range (or io_remap_pfn_range)
then there is are vm_ops attached by default.

gfn_to_pfn: vma 0xffff88022c50d498 start 0x7f4b0de9f000 pgoff 0x0
flags 0x844fb vm_ops 0x0000000000000000 fault 0x0000000000000000 file
0xffff88022e408000 major 250 minor 32

From linux/mm.h:

#define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct
page", just pure PFN */

Stephen.