Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* H. Peter Anvin (hpa@zytor.com) wrote:
> Pavel Machek wrote:
>> On Thu 2008-04-17 16:14:10, Mathieu Desnoyers wrote:
>>> (hopefully finally CCing LKML) :)
>>>
>>> Implements an alternative iret with popf and return so trap and exception
>>> handlers can return to the NMI handler without issuing iret. iret would 
>>> cause
>>> NMIs to be reenabled prematurely. x86_32 uses popf and far return. x86_64 
>>> has to
>>> copy the return instruction pointer to the top of the previous stack, 
>>> issue a
>>> popf, loads the previous esp and issue a near return (ret).
>> sounds expensive. Does it slow down normal loads?
>
> It should *only* be used to return from NMI, #MC or INT3 (breakpoint), 
> which should never happen in normal operation, and even then only when 
> interrupting another NMI or #MC handler.
>
> 	-hpa
>

Just to be clear : the added cost on normal interrupt return is to add a
supplementary test of the thread flags already loaded in registers and
a conditional branch. This is used to detect if we are nested over an
NMI handler. I doubt anyone ever notice an impact caused by this added
test/branch.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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