On 08/13/2009 07:07 AM, Stephen Donnelly wrote:
A less intrusive, but uglier, alternative is to call
qemu_ram_alloc() and them mmap(MAP_FIXED) on top of that.
I did try this, but ended up with a BUG on the host in
/var/lib/dkms/kvm/84/build/x86/kvm_main.c:1266 gfn_to_pfn() on the
line "BUG_ON(!kvm_is_mmio_pfn(pfn));" when the guest accesses the bar.
It looks to me from the call trace like the guest is writing to the
memory, gfn_to_pfn() from mmu_guess_page_from_pte_write() gets
confused because of the mapping.
Inside gfn_to_pfn:
addr = gfn_to_hva(kvm, gfn); correctly returns the host virtual
address of the external memory mapping.
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.
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.
BUG_ON(!kvm_is_mmio_pfn(pfn)) then triggers.
That's correct behaviour. We expect a page that is not controlled by
the kernel here.
Instrumenting inside gfn_to_pfn I see:
gfn_to_pfn: gfn f2010 gpte f2010000 hva 7f3eac2b0000 pfn 0 npages -14
gfn_to_pfn: vma ffff88022142af18 start 7f3eac2b0000 pgoff 0
Any suggestions what should be happening here?
Well, we need to understand how that vma came into being and why pgoff == 0.
--
error compiling committee.c: too many arguments to function
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