linux-kernel - Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host issue

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Date:   Wed, 9 Jan 2019 12:39:15 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Babu Moger <Babu.Moger@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jan H. Schönherr <jschoenh@...zon.de>,
        David Duncan <davdunc@...zon.com>
Subject: Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host
 issue

On Tue, Jan 08, 2019 at 09:39:46PM -0800, Reinette Chatre wrote:
> To clarify please the last sentence applies to real Haswell server CPUs
> (not virtualized as prompted this work, my apologies for the confusion)
> that support Intel RDT but does not have CPUID enumeration for this
> feature. With Vitaly's patch this hardware would still be detected as
> supporting CAT but if CPUID enumeration is moved earlier then from what
> I can tell this hardware would be considered as not supporting the
> feature anymore.

Ok, so hw "forgot" to add CPUID again. And CPU guys should know better
but everytime we hear "important reasons" why they dropped the ball
there.

So, assuming RDT is not going to be supported in a guest, we need a
proper fix to disable it when in a guest. So the RDT init path needs
something like this then:

---
diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index c3a9dc63edf2..1e5a1cb49e9c 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -969,6 +969,9 @@ static int __init resctrl_late_init(void)
 	struct rdt_resource *r;
 	int state, ret;
 
+	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
+		return -ENODEV;
+
 	/*
 	 * Initialize functions(or definitions) that are different
 	 * between vendors here.

-- 
Regards/Gruss,
    Boris.

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