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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Borislav Petkov <bp@alien8.de>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Babu Moger <Babu.Moger@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        =?utf-8?Q?Jan_H_=2E?= =?utf-8?Q?_Sch=C3=B6nherr?= 
        <jschoenh@amazon.de>, David Duncan <davdunc@amazon.com>
Subject: Re: [PATCH] x86/intel_rdt: use rdmsr_safe() to workaround AWS host issue
In-Reply-To: <20181220161722.GD31403@zn.tnic>
References: <20181220134046.7916-1-vkuznets@redhat.com> <20181220161722.GD31403@zn.tnic>
Date:   Thu, 20 Dec 2018 18:31:34 +0100
Message-ID: <87bm5g2jgp.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Borislav Petkov <bp@alien8.de> writes:

> 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?
>

Yes, it does, thanks!

-- 
Vitaly