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Subject: Re: [PATCH v2 1/4] x86/hyper-v: move synic/stimer control structures
 definitions to hyperv-tlfs.h
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <alpine.DEB.2.21.1811281404230.1532@nanos.tec.linutronix.de>
Date:   Wed, 28 Nov 2018 09:55:37 -0800
Cc:     "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Roman Kagan <rkagan@virtuozzo.com>
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To:     Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
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> On Nov 28, 2018, at 5:07 AM, Thomas Gleixner <tglx@linutronix.de> =
wrote:
>=20
> On Wed, 28 Nov 2018, Vitaly Kuznetsov wrote:
>=20
>> Nadav Amit <nadav.amit@gmail.com> writes:
>>=20
>>> On a different note: how come all of the hyper-v structs are not =
marked
>>> with the =E2=80=9Cpacked" attribute?
>>=20
>> "packed" should not be needed with proper padding; I vaguely remember
>> someone (from x86@?) arguing _against_ "packed".
>=20
> 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.
>=20
> 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.