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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