Re: [PATCH 7/7] x86/microcode: Recheck IBRS features on microcode reload

Andrea Arcangeli
2018-01-04T18:34:34Z

On Thu, Jan 04, 2018 at 07:28:58PM +0100, Borislav Petkov wrote:
> On Thu, Jan 04, 2018 at 09:56:48AM -0800, Tim Chen wrote:
> > On new microcode write, check whether IBRS
> > is present by rescanning scattered CPU features.
> >
> > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > ---
> > arch/x86/kernel/cpu/microcode/core.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> > index c4fa4a8..44b9355 100644
> > --- a/arch/x86/kernel/cpu/microcode/core.c
> > +++ b/arch/x86/kernel/cpu/microcode/core.c
> > @@ -40,6 +40,7 @@
> > #include <asm/processor.h>
> > #include <asm/cmdline.h>
> > #include <asm/setup.h>
> > +#include <asm/spec_ctrl.h>
> >
> > #define DRIVER_VERSION "2.2"
> >
> > @@ -444,6 +445,11 @@ static ssize_t microcode_write(struct file *file, const char __user *buf,
> > if (ret > 0)
> > perf_check_microcode();
> >
> > + /* check spec_ctrl capabilities */
> > + mutex_lock(&spec_ctrl_mutex);
> > + init_scattered_cpuid_features(&boot_cpu_data);
>
> No need for that - make a specific function like perf_check_microcode()
> which checks only the IBRS bit and updates stuff accordingly.

It would be better I agree. I've got this:

void spec_ctrl_rescan_cpuid(void)
{
int cpu;

if (use_ibp_disable)
return;
mutex_lock(&spec_ctrl_mutex);
if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL ||
boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
/* detect spec ctrl related cpuid additions */
init_scattered_cpuid_features(&boot_cpu_data);
spec_ctrl_init(&boot_cpu_data);

/*
* The SPEC_CTRL and IBPB_SUPPORT cpuid bits may have
* just been set in the boot_cpu_data, transfer them
* to the per-cpu data too. This must run after
* spec_ctrl_init() to take care of
* setup_force_cpu_cap() too.
*/
if (cpu_has_spec_ctrl())
for_each_online_cpu(cpu)
set_cpu_cap(&cpu_data(cpu),
X86_FEATURE_SPEC_CTRL);
if (boot_cpu_has(X86_FEATURE_IBPB_SUPPORT))
for_each_online_cpu(cpu)
set_cpu_cap(&cpu_data(cpu),
X86_FEATURE_IBPB_SUPPORT);
}
mutex_unlock(&spec_ctrl_mutex);
}

However we've to start somewhere so that is a simpler start..