linux-kernel - Re: [PATCH v4 7/7] x86/speculation: Add basic IBPB (Indirect Branch Prediction Barrier) support

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Date:   Thu, 25 Jan 2018 11:47:35 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        dave.hansen@...el.com, gnomes@...rguk.ukuu.org.uk,
        ashok.raj@...el.com, mingo@...nel.org
Subject: Re: [PATCH v4 7/7] x86/speculation: Add basic IBPB (Indirect
 Branch Prediction Barrier) support

On Thu, 2018-01-25 at 12:41 +0100, Borislav Petkov wrote:
> 
> > +static inline void indirect_branch_prediction_barrier(void)
> > +{
> > +     asm volatile(ALTERNATIVE("",
> > +                              "movl %[msr], %%ecx\n\t"
> > +                              "movl %[val], %%eax\n\t"
> > +                              "movl $0, %%edx\n\t"
> > +                              "wrmsr",
> > +                              X86_FEATURE_IBPB)
> > +                  : : [msr] "i" (MSR_IA32_PRED_CMD),
> > +                      [val] "i" (PRED_CMD_IBPB)
> > +                  : "eax", "ecx", "edx", "memory");
> 
> Lemme paste my simplification suggestion from the other day:
> 
> "Btw, we can simplify this a bit by dropping the inputs and marking the 3
> GPRs as clobbered:
> 
>         alternative_input("",
>                           "mov $0x49, %%ecx\n\t"
>                           "mov $1, %%eax\n\t"
>                           "xor %%edx, %%edx\n\t"
>                           "wrmsr\n\t",
>                           X86_FEATURE_IBPB,
>                           ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));

Yeah, I saw that; sorry for not replying explicitly.

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.
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