Re: R/W HG memory mappings with kvm?

[Avi Kivity <avi@redhat.com>]

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.

>> do_mmap(), but don't use it.  Use mmap() from userspace like everyone else.
>>      
> Of course you are right, gfn_to_pfn is in user space. There is already
> a mapping of the memory to the process (from qemu_ram_mmap), the
> question is how to look it up.
>    

I'm totally confused now.

-- 
error compiling committee.c: too many arguments to function