linux-kernel - Re: [PATCH] x86/srso: Disable the mitigation on unaffected configurations

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Date:   Mon, 14 Aug 2023 13:08:13 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Nikolay Borisov <nik.borisov@...e.com>
Cc:     Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/srso: Disable the mitigation on unaffected
 configurations

On Mon, Aug 14, 2023 at 09:39:11AM +0300, Nikolay Borisov wrote:
> 
> 
> On 13.08.23 г. 13:45 ч., Borislav Petkov wrote:
> > From: "Borislav Petkov (AMD)" <bp@...en8.de>
> > 
> > Skip the srso cmd line parsing which is not needed on Zen1/2 with SMT
> > disabled and with the proper microcode applied (latter should be the
> > case anyway) as those are not affected.
> > 
> > Fixes: 5a15d8348881 ("x86/srso: Tie SBPB bit setting to microcode patch detection")
> > Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> > ---
> >   arch/x86/kernel/cpu/bugs.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index d02f73c5339d..8959a1b9fb80 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -2418,8 +2418,10 @@ static void __init srso_select_mitigation(void)
> >   		 * IBPB microcode has been applied.
> >   		 */
> >   		if ((boot_cpu_data.x86 < 0x19) &&
> > -		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED)))
> > +		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) {
> >   			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
> > +			goto pred_cmd;
> 
> Actually can't you simply return, given that zen1/2 never have the SBPB flag
> set? Only zen3/4 with the updated microcode?

Tangentially, the 'cpu_smt_control == CPU_SMT_DISABLED' check is wrong,
as SMT could still get enabled at runtime and SRSO would be exposed.

Also is there a reason to re-use the hardware SRSO_NO bit rather than
clear the bug bit?  That seems cleaner, then you wouldn't need this
hack:

> > +	if (boot_cpu_has(X86_FEATURE_SRSO_NO))
> > +		return sysfs_emit(buf, "Not affected\n");
> > +

-- 
Josh