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

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Date:   Fri, 30 Nov 2018 14:26:07 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "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>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "x86@...nel.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

On Fri, Nov 30, 2018 at 02:44:54PM +0100, Vitaly Kuznetsov wrote:
> 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.

Looks so indeed.

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

It's Linux guests on Hyper-V that need to be checked.

> >  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.

Indeed.

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

I guess you're right, in the current code the relevant structures are
made sufficiently aligned by other means.  False alarm, sorry.

Roman.