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Date:   Tue, 5 Mar 2019 13:28:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kairui Song <kasong@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        devel@...uxdriverproject.org
Subject: Re: [RFC PATCH] x86, hyperv: fix kernel panic when kexec on HyperV VM

On Wed, Feb 27, 2019 at 10:55:46PM +0800, Kairui Song wrote:
> On Wed, Feb 27, 2019 at 8:02 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Feb 26, 2019 at 11:56:15PM +0800, Kairui Song wrote:
> > >  arch/x86/hyperv/hv_init.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > > index 7abb09e2eeb8..92291c18d716 100644
> > > --- a/arch/x86/hyperv/hv_init.c
> > > +++ b/arch/x86/hyperv/hv_init.c
> > > @@ -406,6 +406,10 @@ void hyperv_cleanup(void)
> > >       /* Reset our OS id */
> > >       wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
> > >
> > > +     /* Cleanup page reference before reset the page */
> > > +     hv_hypercall_pg = NULL;
> > > +     wmb();
> >
> > What do we need that SFENCE for? Any why does it lack a comment?
> 
> Hi, that's for ensuring the hv_hypercall_pg is reset to NULL before
> the following wrmsr call. The wrmsr call will make the pointer address
> invalid.

WRMSR is a serializing instruction (except for TSC_DEADLINE and the
X2APIC).

> I can add some comment in V2 if this is OK.

Barriers must always have a comment.

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