linux-kernel - Re: [PATCH] x86/hyper-v: define structures from TLFS as packed

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Message-ID: <87va4ewug9.fsf@vitty.brq.redhat.com>
Date:   Fri, 30 Nov 2018 14:44:54 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Roman Kagan <rkagan@...tuozzo.com>
Cc:     "kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "x86\@kernel.org" <x86@...nel.org>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/hyper-v: define structures from TLFS as packed

Roman Kagan <rkagan@...tuozzo.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