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

Borislav Petkov
2018-12-20T16:17:29Z

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@redhat.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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