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Date: Wed, 29 Nov 2017 13:31:58 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 4/6] x86/mm/kaiser: Support PCID without INVPCID
Message-ID: <20171129123158.xqgiusjamnt2udag@hirez.programming.kicks-ass.net>
References: <20171129103301.131535445@infradead.org>
 <20171129103512.819130098@infradead.org>
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To: linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>, Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, Rik van Riel <riel@redhat.com>, daniel.gruss@iaik.tugraz.at, hughd@google.com, keescook@google.com, linux-mm@kvack.org, michael.schwarz@iaik.tugraz.at, moritz.lipp@iaik.tugraz.at, richard.fellner@student.tugraz.at, Andy Lutomirski <luto@amacapital.net>

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

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