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SubjectRe: [PATCH 22/43] x86/mm/kaiser: Prepare assembly for entry/exit CR3 switching

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Nov 24, 2017 at 10:14:27AM +0100, Ingo Molnar wrote:
> > Interactions with SWAPGS: previous versions of the KAISER code
> > relied on having per-cpu scratch space to save/restore a register
> > that can be used for the CR3 MOV. The %GS register is used to
> > index into our per-cpu space, so SWAPGS *had* to be done before
> > the CR3 switch. That scratch space is gone now, but the semantic
> > that SWAPGS must be done before the CR3 MOV is retained. This is
> > good to keep because it is not that hard to do and it allows us
> > to do things like add per-cpu debugging information to help us
> > figure out what goes wrong sometimes.
>
> > +.macro SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg:req save_reg:req
> > + movq %cr3, %r\scratch_reg
> > + movq %r\scratch_reg, \save_reg
> > + /*
> > + * Is the switch bit zero? This means the address is
> > + * up in real KAISER patches in a moment.
> > + */
> > + testq $(KAISER_SWITCH_MASK), %r\scratch_reg
> > + jz .Ldone_\@
> > +
> > + ADJUST_KERNEL_CR3 %r\scratch_reg
> > + movq %r\scratch_reg, %cr3
> > +
> > +.Ldone_\@:
> > +.endm
>
> > @@ -1333,6 +1362,7 @@ ENTRY(error_entry)
> > * gsbase and proceed. We'll fix up the exception and land in
> > * .Lgs_change's error handler with kernel gsbase.
> > */
> > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
> > SWAPGS
> > jmp .Lerror_entry_done
> >
>
> > @@ -1343,9 +1373,10 @@ ENTRY(error_entry)
> >
> > .Lerror_bad_iret:
> > /*
> > - * We came from an IRET to user mode, so we have user gsbase.
> > - * Switch to kernel gsbase:
> > + * We came from an IRET to user mode, so we have user
> > + * gsbase and CR3. Switch to kernel gsbase and CR3:
> > */
> > + SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
> > SWAPGS
> >
> > /*
>
> The Changelog states SWAPGS must be done before, yet the code does
> after.

Yes, so this is the SWAPGS that is done before we go back to user-space.

Thanks,

Ingo

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