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

[Borislav Petkov]

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.

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