Re: [kvm-devel] [BUG] Oops with KVM-27

From: Avi Kivity
Date: Fri Jun 15 2007 - 05:07:33 EST

Luca wrote:
>> Got it!
>> The emulator skips the writeback if the old value is unchanged, so the
>> apic doesn't see the write.
>> Forcing the writeback:
>> - if ((d & Mov) || (dst.orig_val != dst.val)) {
>> - if ((d & Mov) || (dst.orig_val != dst.val) || isxchg) {
>> seems to fix the issue :D I'm not sure that fix is correct though.

Good detective work!

> After a bit of thinking: it's correct but removes an optimization;
> furthermore it may miss other instructions that write to memory mapped
> areas.
> A more proper fix should be "force the writeback if dst.ptr is in some
> kind of mmio area".

I think we can just disable the optimization, and (in a separate patch)
add it back in emulator_write_phys(), where we know we're modifying
memory and not hitting mmio.

Incidentally, in Xen, where this code originated from, this emulator is
used only for page table updates (which are always to memory). A
separate emulator is used for mmio. I guess the optimization targets
the vm scanner doing test_and_clear_bit() type of operations against the
page tables' dirty bits.

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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