linux-kernel - Re: [PATCH v5 05/32] x86/CPU/AMD: Handle SME reduction in physical address size

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Message-ID: <aaa52e93-5875-6033-e72f-8fc3de43ca3a@amd.com>
Date:   Thu, 20 Apr 2017 12:29:20 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     <linux-arch@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
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        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 05/32] x86/CPU/AMD: Handle SME reduction in physical
 address size

On 4/20/2017 11:59 AM, Borislav Petkov wrote:
> On Tue, Apr 18, 2017 at 04:17:11PM -0500, Tom Lendacky wrote:
>> When System Memory Encryption (SME) is enabled, the physical address
>> space is reduced. Adjust the x86_phys_bits value to reflect this
>> reduction.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>> ---
>>  arch/x86/kernel/cpu/amd.c |   14 +++++++++++---
>>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> ...
>
>> @@ -622,8 +624,14 @@ static void early_init_amd(struct cpuinfo_x86 *c)
>>
>>  			/* Check if SME is enabled */
>>  			rdmsrl(MSR_K8_SYSCFG, msr);
>> -			if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
>> +			if (msr & MSR_K8_SYSCFG_MEM_ENCRYPT) {
>> +				unsigned int ebx;
>> +
>> +				ebx = cpuid_ebx(0x8000001f);
>> +				c->x86_phys_bits -= (ebx >> 6) & 0x3f;
>> +			} else {
>>  				clear_cpu_cap(c, X86_FEATURE_SME);
>> +			}
>
> Lemme do some simplifying to save an indent level, get rid of local var
> ebx and kill some { }-brackets for a bit better readability:
>
>         if (c->extended_cpuid_level >= 0x8000001f) {
>                 u64 msr;
>
>                 if (!cpu_has(c, X86_FEATURE_SME))
>                         return;
>
>                 /* Check if SME is enabled */
>                 rdmsrl(MSR_K8_SYSCFG, msr);
>                 if (msr & MSR_K8_SYSCFG_MEM_ENCRYPT)
>                         c->x86_phys_bits -= (cpuid_ebx(0x8000001f) >> 6) & 0x3f;
>                 else
>                         clear_cpu_cap(c, X86_FEATURE_SME);
>         }
>

Hmmm... and actually if cpu_has(X86_FEATURE_SME) is true then it's a
given that extended_cpuid_level >= 0x8000001f.  So this can be
simplified to just:

	if (cpu_has(c, X86_FEATURE_SME)) {
		... the rest of your suggestion (minus cpu_has()) ...
	}

Thanks,
Tom