Re: [PATCH v2] x86/bugs: Explicitly clear speculative MSR bits

[Pawan Gupta]

On Mon, Nov 28, 2022 at 11:40:19PM +0100, Borislav Petkov wrote:
> On Mon, Nov 28, 2022 at 02:03:58PM -0800, Pawan Gupta wrote:
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 3e3230cccaa7..cfc2ed2661fc 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -66,7 +66,7 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
> >   */
> >  void write_spec_ctrl_current(u64 val, bool force)
> >  {
> > -	if (this_cpu_read(x86_spec_ctrl_current) == val)
> > +	if (!force && this_cpu_read(x86_spec_ctrl_current) == val)
> >  		return;
> >  	this_cpu_write(x86_spec_ctrl_current, val);
>
> Still looks hacky to me.
>
> I think it would be a lot cleaner if MSR_IA32_SPEC_CTRL gets cleaned of
> the speculation bits in init_speculation_control() which gets run on
> *every* CPU.
>
> So by the time check_bugs() gets to setup stuff, the MSR will be ready
> to go regardless.
>
> I.e., something like this (not supposed to work - just to show what I
> mean):
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 73cc546e024d..367732c92942 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -993,9 +993,19 @@ static void init_speculation_control(struct cpuinfo_x86 *c)
> 	 * Intel CPUs, for finer-grained selection of what's available.
> 	 */
> 	if (cpu_has(c, X86_FEATURE_SPEC_CTRL)) {
> +		u64 msr;
> +
> 		set_cpu_cap(c, X86_FEATURE_IBRS);
> 		set_cpu_cap(c, X86_FEATURE_IBPB);
> 		set_cpu_cap(c, X86_FEATURE_MSR_SPEC_CTRL);
> +
> +		/*
> +		 * Clear speculation control settings from a previous kernel
> +		 * run, i.e., kexec.
> +		 */
> +		rdmsrl(MSR_IA32_SPEC_CTRL, msr);
> +		if (msr & SPEC_CTRL_MASK)
> +			wrmsr (MSR_IA32_SPEC_CTRL, msr & ~SPEC_CTRL_MASK);

Yes thats a cleaner approach, except that the late microcode load will
ruin the MSR:

microcode_reload_late()
  microcode_check()
    get_cpu_cap()
      init_speculation_control()