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Date:   Thu, 14 Jul 2022 07:59:22 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        antonio.gomez.iglesias@linux.intel.com,
        Josh Poimboeuf <jpoimboe@kernel.org>
Subject: Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on
 Enhanced IBRS parts
Message-ID: <20220714145922.vs5to67omdzv6nmp@desk>
References: <0456b35fb9ef957d9a9138e0913fb1a3fd445dff.1657747493.git.pawan.kumar.gupta@linux.intel.com>
 <Ys/7RiC9Z++38tzq@quatroqueijos>
 <YtABEwRnWrJyIKTY@worktop.programming.kicks-ass.net>
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On Thu, Jul 14, 2022 at 01:42:11PM +0200, Peter Zijlstra wrote:
>On Thu, Jul 14, 2022 at 08:17:26AM -0300, Thadeu Lima de Souza Cascardo wrote:
>> On Wed, Jul 13, 2022 at 10:32:37PM -0700, Pawan Gupta wrote:
>> > Currently spectre_v2=ibrs forces write to MSR_IA32_SPEC_CTRL at every
>> > entry and exit. On Enhanced IBRS parts setting MSR_IA32_SPEC_CTRL[IBRS]
>> > only once at bootup is sufficient. MSR write at every kernel entry/exit
>> > incur unnecessary penalty that can be avoided.
>> >
>> > When Enhanced IBRS feature is present, switch from "ibrs" to "auto" mode
>> > so that appropriate mitigation is selected.
>> >
>> > Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
>> > Cc: stable@vger.kernel.org # 5.10+
>> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>> > ---
>> >  arch/x86/kernel/cpu/bugs.c | 6 ++++++
>> >  1 file changed, 6 insertions(+)
>> >
>> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> > index 0dd04713434b..7d7ebfdfbeda 100644
>> > --- a/arch/x86/kernel/cpu/bugs.c
>> > +++ b/arch/x86/kernel/cpu/bugs.c
>> > @@ -1303,6 +1303,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
>> >  		return SPECTRE_V2_CMD_AUTO;
>> >  	}
>> >
>> > +	if (cmd == SPECTRE_V2_CMD_IBRS && boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
>> > +		pr_err("%s selected but CPU supports Enhanced IBRS. Switching to AUTO select\n",
>> > +		       mitigation_options[i].option);
>> > +		return SPECTRE_V2_CMD_AUTO;
>> > +	}
>> > +
>> >  	spec_v2_print_cond(mitigation_options[i].option,
>> >  			   mitigation_options[i].secure);
>> >  	return cmd;
>> >
>> > base-commit: 72a8e05d4f66b5af7854df4490e3135168694b6b
>> > --
>> > 2.35.3
>> >
>> >
>>
>> Shouldn't we just use the mitigation the user asked for if it is still
>> possible? We could add the warning advising the user that a different
>> mitigation could be used instead with less penalty, but if the user asked for
>> IBRS and that is available, it should be used.
>>
>> One of the reasons for that is testing. I know it was useful enough for me and
>> it helped me find some bugs.
>
>Yeah this; if the user asks for IBRS, we should give him IBRS. I hate
>the 'I know better, let me change that for you' mentality.
>
>If you want to do something, print a warning.

Fair enough, I will change that to a warning.

Thanks,
Pawan