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

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