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Message-ID: <1516881500.30244.83.camel@infradead.org>
Subject: Re: [PATCH v4 7/7] x86/speculation: Add basic IBPB (Indirect
 Branch Prediction Barrier) support
From:   David Woodhouse <dwmw2@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     arjan@linux.intel.com, tglx@linutronix.de, karahmed@amazon.de,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        tim.c.chen@linux.intel.com, peterz@infradead.org,
        pbonzini@redhat.com, ak@linux.intel.com,
        torvalds@linux-foundation.org, gregkh@linux-foundation.org,
        dave.hansen@intel.com, gnomes@lxorguk.ukuu.org.uk,
        ashok.raj@intel.com, mingo@kernel.org
In-Reply-To: <20180125115016.ov72ajqb6z7scdrc@pd.tnic>
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Date:   Thu, 25 Jan 2018 11:58:20 +0000
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On Thu, 2018-01-25 at 12:50 +0100, Borislav Petkov wrote:
> On Thu, Jan 25, 2018 at 11:47:35AM +0000, David Woodhouse wrote:
> > I did already do the explicit clobbers now you reminded me how to do
> > them, but I elected not to use ASM_NO_INPUT_CLOBBER() because on
> > balance, losing the MSR_IA32_PRED_CMD / PRED_CMD_IBPB definitions and
> > having to hard-code the numbers was not so much of a win.
>=20
> and I'm thinking exactly the opposite: if an alternative can lose a
> bunch of inputs which don't change, then that's a good thing.

They're immediates, not registers. So it's like the first example in...

void foo(void)
{
	asm volatile ("alternative me away; my numbers are %0 and %1\n"
		=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0:: "i" (3456), "i" (1234));

	asm volatile ("alternative me away; GCC already loaded regs %0 and %1\n"
		=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0:: "c" (3456), "a" (1234));
}

... which gives you...

foo:
.LFB0:
	.cfi_startproc
#APP
# 4 "foo.c" 1
	alternative me away; my numbers are $3456 and $1234

# 0 "" 2
#NO_APP
	movl	$1234, %eax
	movl	$3456, %ecx
#APP
# 7 "foo.c" 1
	alternative me away; GCC already loaded regs %ecx and %eax

# 0 "" 2
#NO_APP
	ret
	.cfi_endproc

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