Re: [PATCH 01/22] x86/srso: Fix srso_show_state() side effect

[Josh Poimboeuf]

On Mon, Aug 21, 2023 at 08:04:16AM +0200, Borislav Petkov wrote:
> On Sun, Aug 20, 2023 at 06:18:58PM -0700, Josh Poimboeuf wrote:
> > Reading the 'spec_rstack_overflow' sysfs file can trigger an unnecessary
> > MSR write, and possibly even a (handled) exception if the microcode
> > hasn't been updated.
> > 
> > Avoid all that by just checking X86_FEATURE_IBPB_BRTYPE instead, which
> > gets set by srso_select_mitigation() if the updated microcode exists.
> > 
> > Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index f081d26616ac..bdd3e296f72b 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -2717,7 +2717,7 @@ static ssize_t srso_show_state(char *buf)
> >
> 
> Please put here a comment - something along the lines of:
> 
> "X86_FEATURE_IBPB_BRTYPE gets set as a result of the presence of the
> needed microcode so checking that is equivalent."
> 
> so that it is clear why it is ok to check this feature bit.

I could do that, but this check ends up getting replaced by a later
patch anyway.

Would you want this comment in srso_select_mitigation()?  After the next
patch it has:

  bool has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE);

Though that seems clear to me already.

-- 
Josh