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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Message-ID: <870ff11e-2ba0-22ce-51d5-450124ae1111@amd.com>
Date:   Sat, 2 Sep 2017 21:34:48 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     brijesh.singh@....com, 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>, 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 9/1/17 10:21 PM, Andy Lutomirski wrote:
> 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.

Thanks for the suggestion, we could call it DEFINE_PER_CPU_UNENCRYPTED.
I will use it in next rev.

-Brijesh