linux-kernel - Re: [PATCH v2 1/4] x86/hyper-v: move synic/stimer control structures definitions to hyperv-tlfs.h

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Date:   Wed, 28 Nov 2018 09:55:37 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH v2 1/4] x86/hyper-v: move synic/stimer control structures
 definitions to hyperv-tlfs.h

> On Nov 28, 2018, at 5:07 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Wed, 28 Nov 2018, Vitaly Kuznetsov wrote:
> 
>> Nadav Amit <nadav.amit@...il.com> writes:
>> 
>>> On a different note: how come all of the hyper-v structs are not marked
>>> with the “packed" attribute?
>> 
>> "packed" should not be needed with proper padding; I vaguely remember
>> someone (from x86@?) arguing _against_ "packed".
> 
> Packed needs to be used, when describing fixed format data structures in
> hardware or other ABIs, so the compiler cannot put alignment holes into
> them.
> 
> Using packed for generic data structures might result in suboptimal layouts
> and prevents layout randomization.

Right, I forgot about the structs randomization. So at least for it, the
attribute should be needed.

To prevent conflicts, I think that this series should also add the
attribute in a first patch, which would be tagged for stable.