Re: [PATCH] i386: improve double fault handling

["H. Peter Anvin" <hpa@zytor.com>]

Jan Beulich wrote:
>>>> Joerg Roedel <joro@8bytes.org> 23.07.08 23:43 >>>
>> On Fri, Jul 18, 2008 at 01:30:42PM +0100, Jan Beulich wrote:
>>> Make the double fault handler use CPU-specific stacks. Add some
>>> abstraction to simplify future change of other exception handlers to go
>>> through task gates.
>> What is the benefit of exception handlers going through task gates?
>> Hardware task switches are not very well supported in virtualization
>> (e.g. its has issues in KVM and is also not in Xen for a long time).
> 
> The main goal is to get to a different stack. While at present this is done
> only for the double fault, I think generally NMI and MCE should also do
> so, as they may be caused by a stack access (see x86-64, which runs
> them on IST stacks), and hence continuing to run on that same stack
> may not allow the exception to be handled.

NMI, MCE and #DF are the obvious candidates.

Now, keep in mind TSSes have to be prepared per-CPU, since they get 
marked "busy" when in use, so it's a bit of a nontrivial undertaking.

	-hpa