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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Roman Kagan <rkagan@virtuozzo.com>
Cc:     "kvm\@vger.kernel.org" <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "x86\@kernel.org" <x86@kernel.org>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86/hyper-v: define structures from TLFS as packed
In-Reply-To: <20181130131122.GA2358@rkaganb.sw.ru>
References: <20181130121511.12331-1-vkuznets@redhat.com> <20181130131122.GA2358@rkaganb.sw.ru>
Date:   Fri, 30 Nov 2018 14:44:54 +0100
Message-ID: <87va4ewug9.fsf@vitty.brq.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Roman Kagan <rkagan@virtuozzo.com> writes:

> On Fri, Nov 30, 2018 at 01:15:11PM +0100, Vitaly Kuznetsov wrote:
>> Without 'packed' compiler is free to add optimization paddings and re-order
>> structure fields for randomization/optimization. And structures from
>> hyperv-tlfs.h are used for hypervisor-guest communication, we need to
>> ultimately forbid such practices.
>
> Note that __packed also reduces the structure alignment to 1, which is
> not necessarily what you want.
>
> E.g. some of these structures are passed by pointer to the hypercall,
> which requires its arguments to be 8byte-aligned.

Hm,

I thought we always take precautions for Hyper-V hypercall arguments, in
particular

PV IPI/TLB flush use pre-allocated hyperv_pcpu_input_arg,
hv_post_message() uses pre-allocated message page, other call sites use
fast hypercalls where we use registers.

I also checked this patch before sending out, WS2016 guest boots without
issues. Any particular places you're worried about?

>  I'm also not sure
> that passing unaligned argument to [rw]msr is ok, need to
> double-check.

My understanding is that rdmsr/wrmsr instuctions are registers-only.

We can, of course, just add __aligned(8) to some structures but I'd like
to find the reason first.

-- 
Vitaly