Re: [PATCH 02/24] x86/mm/kaiser: Prepare the x86/entry assembly code for entry/exit CR3 switching

Peter Zijlstra
2017-11-27T21:01:02Z

On Mon, Nov 27, 2017 at 06:33:39PM +0100, Thomas Gleixner wrote:
> On Mon, 27 Nov 2017, Peter Zijlstra wrote:
> > On Mon, Nov 27, 2017 at 11:49:01AM +0100, Ingo Molnar wrote:
> > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > > index caf74a1bb3de..df0152bee8a8 100644
> >
> > > @@ -198,6 +201,14 @@ ENTRY(entry_SYSCALL_64)
> > >
> > > swapgs
> > > movq %rsp, PER_CPU_VAR(rsp_scratch)
> > > +
> > > + /*
> > > + * The kernel CR3 is needed to map the process stack, but we
> > > + * need a scratch register to be able to load CR3. %rsp is
> > > + * clobberable right now, so use it as a scratch register.
> > > + * %rsp will look crazy here for a couple instructions.
> > > + */
> > > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
> > > movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
> > >
> > > /* Construct struct pt_regs on stack */
> >
> > I think we can do away with this one, on account of never getting here
> > because of entry_SYSCALL_64_trampoline.
>
> It's on my list already. About 5 people noticed :)

Hey, at least we all agree ;-)