Re: [PATCH v2] x86/ibpb: Skip IBPB when we switch back to same user process

[Ingo Molnar]

* Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:

> Thanks to the reviewers and Andy Lutomirski for the suggestion of
> using ctx_id which got rid of the problem of mm pointer recycling.
> Here's an update of this patch based on Andy's suggestion.
> 
> We could switch to a kernel idle thread and then back to the original
> process such as:
> process A -> idle -> process A
> 
> In such scenario, we do not have to do IBPB here even though the process is
> non-dumpable, as we are switching back to the same process after
> an hiatus.
> 
> We track the last mm user context id before we switch to init_mm by calling
> leave_mm when tlb_defer_switch_to_init_mm returns false (pcid available).
> 
> The cost is to have an extra u64 mm context id to track the last mm we were using before
> switching to the init_mm used by idle.  Avoiding the extra IBPB
> is probably worth the extra memory for this common scenario.
> 
> For those cases where tlb_defer_switch_to_init_mm returns true (non pcid),
> lazy tlb will defer switch to init_mm, so we will not be changing
> the mm for the process A -> idle -> process A switch. So
> IBPB will be skipped for this case.
> 
> v2:
> 1. Save last user context id instead of last user mm to avoid the problem of recycled mm
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/tlbflush.h |  2 ++
>  arch/x86/mm/tlb.c               | 23 ++++++++++++++++-------
>  2 files changed, 18 insertions(+), 7 deletions(-)

What tree is this patch against? It doesn't apply to linus's latest, nor to 
tip:master.

Thanks,

	Ingo