linux-kernel - Re: [PATCH 4/6] x86/mm/kaiser: Support PCID without INVPCID

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Date:   Wed, 29 Nov 2017 13:31:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 4/6] x86/mm/kaiser: Support PCID without INVPCID

On Wed, Nov 29, 2017 at 11:33:05AM +0100, Peter Zijlstra wrote:
> @@ -220,7 +215,27 @@ For 32-bit we have the following convent
>  .macro SWITCH_TO_USER_CR3 scratch_reg:req
>  	STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
>  	mov	%cr3, \scratch_reg
> -	ADJUST_USER_CR3 \scratch_reg
> +
> +	/*
> +	 * Test if the ASID needs a flush.
> +	 */
> +	push	\scratch_reg			/* preserve CR3 */

So I was just staring at disasm of a few functions and I noticed this
one reads like push, while others read like pushq.

So does the stupid assembler thing really do a 32bit push if you provide
it with a 64bit register?

> +	andq	$(0x7FF), \scratch_reg		/* mask ASID */
> +	bt	\scratch_reg, PER_CPU_VAR(user_asid_flush_mask)
> +	jnc	.Lnoflush_\@
> +
> +	/* Flush needed, clear the bit */
> +	btr	\scratch_reg, PER_CPU_VAR(user_asid_flush_mask)
> +	pop	\scratch_reg			/* original CR3 */
> +	jmp	.Ldo_\@
> +
> +.Lnoflush_\@:
> +	pop	\scratch_reg			/* original CR3 */
> +	ALTERNATIVE "", "bts $63, \scratch_reg", X86_FEATURE_PCID
> +
> +.Ldo_\@:
> +	/* Flip the PGD and ASID to the user version */
> +	orq     $(KAISER_SWITCH_MASK), \scratch_reg
>  	mov	\scratch_reg, %cr3
>  .Lend_\@:
>  .endm