Re: [PATCH v3 04/12] Add "handle page fault" PV helper.

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

On 01/19/2010 09:44 AM, Gleb Natapov wrote:
> 
> Yes it can be done this way and I'll look into it once more. Using
> exception vector is more convenient for three reasons: it allows to pass
> additional data in error code, it doesn't require guest to issue EOI,
> exception can be injected when interrupts are disabled by a guest. The
> last one is not important for now since host doesn't inject notifications
> when interrupts are disabled currently. Having Intel allocate one
> exception vector for hypervisor use would be really nice though.
> 

That's probably not going to happen, for the rather obvious reason: *you
already have 224 of them*.

You seem to be thinking here that vectors 0-31 have to be exceptions and
32-255 have to be interrupts.  *There is no such distinction*; the only
thing special about 0-31 is that we (Intel) reserve the right to control
the assignments; for 32-255 the platform and OS control the assignment.

You can have the guest OS take an exception on a vector above 31 just
fine; you just need it to tell the hypervisor which vector it, the OS,
assigned for this purpose.

	-hpa