linux-kernel - Re: [PATCH 05/30] x86, kaiser: prepare assembly for entry/exit CR3 switching

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Date:   Mon, 20 Nov 2017 13:17:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 05/30] x86, kaiser: prepare assembly for entry/exit CR3
 switching

On Fri, 10 Nov 2017, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> This is largely code from Andy Lutomirski.  I fixed a few bugs
> in it, and added a few SWITCH_TO_* spots.
> 
> KAISER needs to switch to a different CR3 value when it enters
> the kernel and switch back when it exits.  This essentially
> needs to be done before leaving assembly code.
> 
> This is extra challenging because the switching context is
> tricky: the registers that can be clobbered can vary.  It is also
> hard to store things on the stack because there is an established
> ABI (ptregs) or the stack is entirely unsafe to use.

Changelog nitpicking starts here

> This patch establishes a set of macros that allow changing to

s/This patch establishes/Establish/

> the user and kernel CR3 values.
> 
> 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

s/our/the/

> 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

s/us//

> to do things like add per-cpu debugging information to help us
> figure out what goes wrong sometimes.

the part after 'information' is fairy tale mode and redundant. Debugging
information says it all, right?

> What this does in the NMI code is worth pointing out.  NMIs
> can interrupt *any* context and they can also be nested with
> NMIs interrupting other NMIs.  The comments below
> ".Lnmi_from_kernel" explain the format of the stack during this
> situation.  Changing the format of this stack is not a fun
> exercise: I tried.  Instead of storing the old CR3 value on the
> stack, this patch depend on the *regular* register save/restore
> mechanism and then uses %r14 to keep CR3 during the NMI.  It is
> callee-saved and will not be clobbered by the C NMI handlers that
> get called.

  The comments below ".Lnmi_from_kernel" explain the format of the stack
  during this situation. Changing this stack format is too complex and
  risky, so the following solution has been used:

  Instead of storing the old CR3 value on the stack, depend on the regular
  register save/restore mechanism and use %r14 to hold CR3 during the
  NMI. r14 is callee-saved and will not be clobbered by the C NMI handlers
  that get called.

End of nitpicking

> +.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.

  	 * If the switch bit is zero, CR3 points at the kernel page tables
	 * already.
Hmm?

>  /*
> @@ -1189,6 +1201,7 @@ ENTRY(paranoid_exit)
>  	testl	%ebx, %ebx			/* swapgs needed? */
>  	jnz	.Lparanoid_exit_no_swapgs
>  	TRACE_IRQS_IRETQ
> +	RESTORE_CR3	%r14

You have the named macro arguments everywhere, just not here.

Other than that.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>