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

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Date:   Thu, 20 Dec 2018 17:17:22 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Babu Moger <Babu.Moger@....com>
Cc:     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 Thu, Dec 20, 2018 at 02:40:46PM +0100, Vitaly Kuznetsov wrote:
> It was found that AWS x1 instances (Xen-based) lack xen.git commit
> 1f1d183d4900 (x86/HVM: don't give the wrong impression of WRMSR succeeding)
> and because of that the wrmsr_safe() check in cache_alloc_hsw_probe()
> doesn't help: the consequent rdmsr() blows up with
> 
>  unchecked MSR access error: RDMSR from 0xc90 at rIP:
>    0xffffffff88c5bba3 (native_read_msr+0x3/0x30)
> 
> The issue should definitely get fixed on AWS side. We can, however, simply
> workaround this in Linux and live happily after.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kernel/cpu/intel_rdt.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel_rdt.c b/arch/x86/kernel/cpu/intel_rdt.c
> index 44272b7107ad..0acee6cd07a8 100644
> --- a/arch/x86/kernel/cpu/intel_rdt.c
> +++ b/arch/x86/kernel/cpu/intel_rdt.c
> @@ -213,7 +213,8 @@ static inline void cache_alloc_hsw_probe(void)
>  
>  	if (wrmsr_safe(IA32_L3_CBM_BASE, max_cbm, 0))
>  		return;
> -	rdmsr(IA32_L3_CBM_BASE, l, h);
> +	if (rdmsr_safe(IA32_L3_CBM_BASE, &l, &h))
> +		return;
>  
>  	/* If all the bits were set in MSR, return success */
>  	if (l != max_cbm)
> -- 

Does the below hunk work too?

Reinette, Babu: is there any reason for rdt_init_res_defs() and
check_quirks() to run before get_rdt_resources() which looks at CPUID
bits?

If not, we need to do something like below, for guests.

Thx.

---
diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index c3a9dc63edf2..64cb0fb31862 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -969,17 +969,13 @@ static int __init resctrl_late_init(void)
 	struct rdt_resource *r;
 	int state, ret;
 
-	/*
-	 * Initialize functions(or definitions) that are different
-	 * between vendors here.
-	 */
+	if (!get_rdt_resources())
+		return -ENODEV;
+
 	rdt_init_res_defs();
 
 	check_quirks();
 
-	if (!get_rdt_resources())
-		return -ENODEV;
-
 	rdt_init_padding();
 
 	state = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,



-- 
Regards/Gruss,
    Boris.

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