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Message-ID: <1517959878.3677.54.camel@infradead.org>
Subject: Re: [PATCH 2/2] x86/speculation: Simplify
 indirect_branch_prediction_barrier()
From:   David Woodhouse <dwmw2@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, tim.c.chen@linux.intel.com,
        pjt@google.com, jikos@kernel.org, gregkh@linux-foundation.org,
        dave.hansen@intel.com, riel@redhat.com, luto@amacapital.net,
        torvalds@linux-foundation.org, ak@linux.intel.com,
        keescook@google.com, peterz@infradead.org
In-Reply-To: <20180206232514.qcy4y3dzfkjo3xdg@treble>
References: <20180126121139.31959-1-bp@alien8.de>
         <20180126121139.31959-4-bp@alien8.de>
         <1516970011.30244.223.camel@infradead.org>
         <20180126132431.fsbd3c3g2yreazy6@pd.tnic>
         <1516983879.30244.236.camel@infradead.org>
         <20180126164746.dpo7dswid5tjk2tz@pd.tnic>
         <20180126200616.5xfn244uzeu7ptyo@pd.tnic>
         <20180126200813.cignvfovk2dhlzbh@pd.tnic>
         <1517946292.3677.22.camel@infradead.org>
         <20180206232514.qcy4y3dzfkjo3xdg@treble>
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Date:   Tue, 06 Feb 2018 23:31:18 +0000
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On Tue, 2018-02-06 at 17:25 -0600, Josh Poimboeuf wrote:
> On Tue, Feb 06, 2018 at 07:44:52PM +0000, David Woodhouse wrote:
> >=20
> > On Fri, 2018-01-26 at 21:08 +0100, Borislav Petkov wrote:
> > >=20
> > > Make it all a function which does the WRMSR instead of having a hairy
> > > inline asm.
> > ...
> >=20
> > >=20
> > > +	alternative_input("",
> > > +			=C2=A0"call __ibp_barrier",
> > > +			=C2=A0X86_FEATURE_IBPB,
> > > +			=C2=A0ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> > > =C2=A0}
> > Dammit. I know the best time to comment is *before* I add my own sign-
> > off to it and before Linus has merged it but... I think this is broken.
> >=20
> > If you're calling a C function then you have to mark *all* the call-
> > clobbered registers as, well, clobbered.
> >=20
> > If you really really really want to *call* something out of line, then
> > it would need to be implemented in asm.
>
> Hm.=C2=A0=C2=A0In theory I agree this seems like a bug.=C2=A0=C2=A0On x86=
_64 I believe we
> would need to mark the following registers as clobbered: r8-r11, ax, cx,
> dx, si, di, plus "memory" and "cc".
>=20
> But I'm scratching my head a bit, because we seem to have this bug all
> over the kernel.=C2=A0=C2=A0(Grep for ASM_CALL_CONSTRAINT to see them.)
>=20
> Many of those inline asm calls have been around a long time.=C2=A0=C2=A0S=
o why
> hasn't it ever bitten us?

How many are actually calling C functions, not asm or other special
cases like firmware entry points?

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