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From:   Kairui Song <kasong@redhat.com>
Date:   Tue, 5 Mar 2019 21:34:13 +0800
Message-ID: <CACPcB9c=M26J8V7S_SzuccCTcPkr1DOUqK6w01esieJYYeBpEA@mail.gmail.com>
Subject: Re: [PATCH v2] x86, hyperv: fix kernel panic when kexec on HyperV
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        devel@linuxdriverproject.org
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On Tue, Mar 5, 2019 at 8:33 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Mar 05, 2019 at 08:17:03PM +0800, Kairui Song wrote:
> > diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> > index 7abb09e2eeb8..34aa1e953dfc 100644
> > --- a/arch/x86/hyperv/hv_init.c
> > +++ b/arch/x86/hyperv/hv_init.c
> > @@ -406,6 +406,12 @@ void hyperv_cleanup(void)
> >       /* Reset our OS id */
> >       wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
> >
> > +     /* Cleanup hypercall page reference before reset the page */
> > +     hv_hypercall_pg = NULL;
> > +
> > +     /* Make sure page reference is cleared before wrmsr */
>
> This comment forgets to tell us who cares about this. And why the wrmsr
> itself isn't serializing enough.
>
> > +     wmb();
> > +
> >       /* Reset the hypercall page */
> >       hypercall_msr.as_uint64 = 0;
> >       wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
>
> That looks like a fake MSR; and you're telling me that VMEXIT doesn't
> serialize?

Thanks for the review, seem I being a bit paranoid on this. Will drop
it and send a v3 if no one has any other complaint.

--
Best Regards,
Kairui Song