linux-kernel - Re: [PATCH v9 7/8] x86/cpu: Support AMD Automatic IBRS

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Date:   Sat, 25 Feb 2023 00:23:02 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Kim Phillips <kim.phillips@....com>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Alexey Kardashevskiy <aik@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 7/8] x86/cpu: Support AMD Automatic IBRS

On Fri, Feb 24, 2023 at 11:51:17PM +0100, Borislav Petkov wrote:
> Or, actually, we should simply write it again because it is the init
> path and not really a hot path but it should damn well make sure that
> that bit gets set.

Yeah, we have this fancy msr_set_bit() interface which saves us the MSR
write when not needed. And it also tells us that. :-)

So we can do:

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 380753b14cab..2aa089aa23db 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -996,6 +996,12 @@ static void init_amd(struct cpuinfo_x86 *c)
 		msr_set_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);
 
 	check_null_seg_clears_base(c);
+
+	if (cpu_has(c, X86_FEATURE_AUTOIBRS)) {
+		int ret = msr_set_bit(MSR_EFER, _EFER_AUTOIBRS);
+
+		pr_info("%s: CPU%d, ret: %d\n", __func__, smp_processor_id(), ret);
+	}
 }
 
 #ifdef CONFIG_X86_32

---

and the output looks like this:

[    3.046607] x86: Booting SMP configuration:
[    3.046609] .... node  #0, CPUs:          #1
[    2.874768] init_amd: CPU1, ret: 0
[    3.046873]    #2
[    2.874768] init_amd: CPU2, ret: 0
[    3.049155]    #3
[    2.874768] init_amd: CPU3, ret: 0
[    3.050834]    #4
[    2.874768] init_amd: CPU4, ret: 0
...

which says that the bit was already set - which confirms the
trampoline setting thing.

And doing the write again serves as a guard when in the future we decide
to not set EFER anymore - I doubt it - but we can't allow ourselves to
not set the autoibrs bit so one more RDMSR on init doesn't matter.

Proper patch tomorrow.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette