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

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Message-ID: <87bm68i099.fsf@vitty.brq.redhat.com>
Date:   Thu, 29 Nov 2018 12:36:18 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...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\@vger.kernel.org" <kvm@...r.kernel.org>,
        Radim Krčmář 
        <rkrcmar@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86\@kernel.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

Nadav Amit <nadav.amit@...il.com> writes:

>> 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.
>

Not sure when randomization.s used but Hyper-V drivers will of course be
utterly broken with it.

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

As the patchset doesn't add new definitions and as Paolo already queued
it I'd go with a follow-up patch adding "packed" to all hyperv-tlfs.h
structures. The question is how to avoid conflicts when Linus will be
merging this. We can do:
- Topic branch in kvm
- Send the patch to x86, make topic branch and reabse kvm
- Send the patch to kvm
- ... ?

Paolo/Thomas, what would be your preference?

-- 
Vitaly