linux-kernel - Re: [RFC Part1 PATCH v3 16/17] X86/KVM: Provide support to create Guest and HV shared per-CPU variables

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Date:   Fri, 1 Sep 2017 20:21:52 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     Borislav Petkov <bp@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        kvm list <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Piotr Luc <piotr.luc@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Laura Abbott <labbott@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Dave Airlie <airlied@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [RFC Part1 PATCH v3 16/17] X86/KVM: Provide support to create
 Guest and HV shared per-CPU variables

On Fri, Sep 1, 2017 at 3:52 PM, Brijesh Singh <brijesh.singh@....com> wrote:
> Hi Boris,
>
> On 08/30/2017 12:46 PM, Borislav Petkov wrote:
>>
>> On Wed, Aug 30, 2017 at 11:18:42AM -0500, Brijesh Singh wrote:
>>>
>>> I was trying to avoid mixing early and no-early set_memory_decrypted()
>>> but if
>>> feedback is: use early_set_memory_decrypted() only if its required
>>> otherwise
>>> use set_memory_decrypted() then I can improve the logic in next rev.
>>> thanks
>>
>>
>> Yes, I think you should use the early versions when you're, well,
>> *early* :-) But get rid of that for_each_possible_cpu() and do it only
>> on the current CPU, as this is a per-CPU path anyway. If you need to
>> do it on *every* CPU and very early, then you need a separate function
>> which is called in kvm_smp_prepare_boot_cpu() as there you're pre-SMP.
>>
>
> I am trying to implement your feedback and now remember why I choose to
> use early_set_memory_decrypted() and for_each_possible_cpu loop. These
> percpu variables are static. Hence before clearing the C-bit we must
> perform the in-place decryption so that original assignment is preserved
> after we change the C-bit. Tom's SME patch [1] added sme_early_decrypt()
> -- which can be used to perform the in-place decryption but we do not have
> similar routine for non-early cases. In order to address your feedback,
> we have to add similar functions. So far, we have not seen the need for
> having such functions except this cases. The approach we have right now
> works just fine and not sure if its worth adding new functions.
>
> Thoughts ?
>
> [1] Commit :7f8b7e7 x86/mm: Add support for early encryption/decryption of
> memory

Shouldn't this be called DEFINE_PER_CPU_UNENCRYPTED?  ISTM the "HV
shared" bit is incidental.