linux-kernel - Re: [PATCH] x86/speculation: Add support for STIBP always-on preferred mode

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Date:   Tue, 11 Dec 2018 23:22:59 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Lendacky, Thomas" <Thomas.Lendacky@....com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jiri Kosina <jkosina@...e.cz>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH] x86/speculation: Add support for STIBP always-on
 preferred mode

On Tue, Dec 11, 2018 at 10:17:30PM +0000, Lendacky, Thomas wrote:
> If using just SPECTRE_V2_USER_STRICT then the code in stibp_state() would
> have to be able differentiate between the case where the mode was switched
> because of X86_FEATURE_AMD_STIBP_ALWAYS_ON vs a kernel command line with
> "spectre_v2_user=on". I could always set and use a static variable in the
> file just for the stibp_state() case.

Does it matter on X86_FEATURE_AMD_STIBP_ALWAYS_ON CPUs?

I mean, we want STIBP to be always on there so you can do:

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 25e914f77bb8..d14860d1cf9c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1089,7 +1089,10 @@ static char *stibp_state(void)
 	case SPECTRE_V2_USER_NONE:
 		return ", STIBP: disabled";
 	case SPECTRE_V2_USER_STRICT:
-		return ", STIBP: forced";
+		if (boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
+			return ", STIBP: always on";
+		else
+			return ", STIBP: forced";
 	case SPECTRE_V2_USER_PRCTL:
 	case SPECTRE_V2_USER_SECCOMP:
 		if (static_key_enabled(&switch_to_cond_stibp))

so if user has booted with spectre_v2_user=on, we say, "oh well, it is always
enabled anyway"... or?

> I'll give it a bit of time and see if there's any other discussion and
> re-submit without the new SPECTRE_V2_USER_STRICT_PREFERRED value.

Sure.

-- 
Regards/Gruss,
    Boris.

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