On Mon, Aug 31, 2009 at 8:44 PM, Avi Kivity<avi@xxxxxxxxxx> wrote:
> On 08/31/2009 01:33 AM, Stephen Donnelly wrote:
>>
>>> 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.
>>>
>>
>> That may be true for COW pages, which are main memory, but I don't
>> think it is true for device drivers.
>>
>
> No, COW pages have no linear pfn mapping. It's only true for
> remap_pfn_range).
>
>> In a device driver the mmap function receives the vma from the OS. The
>> vm_pgoff field contains the offset area in the file. For drivers this
>> is used to determine where to start the map compared to the io base
>> address.
>>
>> If the driver is mapping io memory to user space it calls
>> io_remap_pfn_range with the pfn for the io memory. The remap_pfn_range
>> call sets the VM_IO and VM_PFNMAP bits in vm_flags. It does not alter
>> the vm_pgoff value.
>>
>> A simple example is hpet_mmap() in drivers/char/hpet.c, or
>> mbcs_gscr_mmap() in drivers/char/mbcs.c.
>>
>
> io_remap_pfn_range() is remap_pfn_range(), which has this:
>
> if (addr == vma->vm_start && end == vma->vm_end) {
> vma->vm_pgoff = pfn;
> vma->vm_flags |= VM_PFN_AT_MMAP;
> }
>
> So remap_pfn_range() will alter the pgoff.
Aha! We are looking at different kernels. I should have mentioned I
was looking at 2.6.28. In mm/memory.c remap_pfn_range() this has:
* There's a horrible special case to handle copy-on-write
* behaviour that some programs depend on. We mark the "original"
* un-COW'ed pages by matching them up with "vma->vm_pgoff".
*/
if (is_cow_mapping(vma->vm_flags)) {
if (addr != vma->vm_start || end != vma->vm_end)
return -EINVAL;
vma->vm_pgoff = pfn;
}
The macro is:
static inline int is_cow_mapping(unsigned int flags)
{
return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}
Because my vma is marked shared, this clause does not operate and
vm_pgoff is not modified (it is still 0).
> I'm totally confused now.
Sorry about that. The issue is the BUG in gfn_to_pgn where the pfn is
not calculated correctly after looking up the vma.
I still don't see how to get the physical address from the vma, since
vm_pgoff is zero, and the vm_ops are not filled. The vma does not seem
to store the physical base address.
Regards,
Stephen.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
