On 08/27/2009 05:34 AM, Stephen Donnelly wrote:
On Mon, Aug 24, 2009 at 4:55 PM, Avi Kivity<avi@xxxxxxxxxx> wrote:
On 08/24/2009 12:59 AM, Stephen Donnelly wrote:
On Thu, Aug 20, 2009 at 12:14 AM, Avi Kivity<avi@xxxxxxxxxx> 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.
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