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SubjectRe: [PATCH 2/5] x86/entry/64, entry: Set up a valid sysenter stack and prepare for 32-bit merge
On Thu, Jul 23, 2015 at 9:10 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
> On Thu, 23 Jul 2015 09:04:38 -0700
> Andy Lutomirski <luto@amacapital.net> wrote:
>
>> On Thu, Jul 23, 2015 at 9:01 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>> > On Thu, 23 Jul 2015 08:31:40 -0700
>> > Andy Lutomirski <luto@kernel.org> wrote:
>> >
>> >> Oddly, 64-bit kernels already allocate a percpu sysenter stack, but
>> >> they don't enable it. Enable the stack and tweak the rest of the
>> >> sysenter setup code to be similar to the 32-bit version.
>> >
>> > I'm curious. Did you do any investigation into the 64bit code to why it
>> > wasn't set? Do you know if that was just overlooked with some of the
>> > merging between i386 and x86_64 systems?
>> >
>> > I'm not asking you to do it if you have not, but if you have, I think
>> > it would be more comforting to know that it was just overlooked than
>> > there being some other subtle reason.
>>
>> I think it's just that it was never necessary. #DB has always used
>> IST on x86_64 (IIRC it was like that in the very first commit), so the
>> sysenter stack was never actually referenced.
>>
>
> Hmm, should this be commented? If it is never referenced, I could
> imagine that would add some confusion to future reviewers of this code.
>

It is referenced by the SWAPGS_UNSAFE_STACK -> SWAPGS thing, and I'm
planning on referencing it more by removing #DB from the debug stack
soon (patches written but not tested), so the window of confusion
should be short.

--Andy


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