linux-kernel - Re: [PATCH v9 04/38] x86/CPU/AMD: Add the Secure Memory Encryption CPU feature

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Message-ID: <df089d57-3785-c669-6c3b-6f90f77c3658@amd.com>
Date:   Tue, 11 Jul 2017 10:14:34 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>
Cc:     linux-arch <linux-arch@...r.kernel.org>, linux-efi@...r.kernel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        the arch/x86 maintainers <x86@...nel.org>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kasan-dev@...glegroups.com, xen-devel@...ts.xen.org,
        Linux-MM <linux-mm@...ck.org>, iommu@...ts.linux-foundation.org,
        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Joerg Roedel <joro@...tes.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dave Young <dyoung@...hat.com>, Rik van Riel <riel@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v9 04/38] x86/CPU/AMD: Add the Secure Memory Encryption
 CPU feature

On 7/11/2017 12:56 AM, Borislav Petkov wrote:
> On Tue, Jul 11, 2017 at 01:07:46AM -0400, Brian Gerst wrote:
>>> If I make the scattered feature support conditional on CONFIG_X86_64
>>> (based on comment below) then cpu_has() will always be false unless
>>> CONFIG_X86_64 is enabled. So this won't need to be wrapped by the
>>> #ifdef.
>>
>> If you change it to use cpu_feature_enabled(), gcc will see that it is
>> disabled and eliminate the dead code at compile time.
> 
> Just do this:
> 
>         if (cpu_has(c, X86_FEATURE_SME)) {
> 	       if (IS_ENABLED(CONFIG_X86_32)) {
>                         clear_cpu_cap(c, X86_FEATURE_SME);
> 	       } else {
> 		       u64 msr;
> 
> 		       /* Check if SME is enabled */
> 	              rdmsrl(MSR_K8_SYSCFG, msr);
> 	              if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
> 	                      clear_cpu_cap(c, X86_FEATURE_SME);
> 	       }
>         }
> 
> so that it is explicit that we disable it on 32-bit and we can save us
> the ifdeffery elsewhere.

I'll use this method for the change and avoid the #ifdefs.

Thanks,
Tom

> 
> Thanks.
>