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From:   Sunil Muthuswamy <sunilmut@microsoft.com>
To:     Wei Liu <wei.liu@kernel.org>, vkuznets <vkuznets@redhat.com>
CC:     Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
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        Michael Kelley <mikelley@microsoft.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Nuno Das Neves <Nuno.Das@microsoft.com>,
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        Haiyang Zhang <haiyangz@microsoft.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: RE: [EXTERNAL] Re: [PATCH RFC v1 08/18] x86/hyperv: handling
 hypercall page setup for root
Thread-Topic: [EXTERNAL] Re: [PATCH RFC v1 08/18] x86/hyperv: handling
 hypercall page setup for root
Thread-Index: AQHWi0uJDiA8iBMLkEafMqtOkreLY6lpgb0AgAJJU9A=
Date:   Wed, 16 Sep 2020 21:34:58 +0000
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>=20
> On Tue, Sep 15, 2020 at 12:32:29PM +0200, Vitaly Kuznetsov wrote:
> > Wei Liu <wei.liu@kernel.org> writes:
> >
> > > When Linux is running as the root partition, the hypercall page will
> > > have already been setup by Hyper-V. Copy the content over to the
> > > allocated page.
> >
> > And we can't setup a new hypercall page by writing something different
> > to HV_X64_MSR_HYPERCALL, right?
> >
>=20
> My understanding is that we can't, but Sunil can maybe correct me.

That is correct. For root partition, the hypervisor has already allocated t=
he
hypercall page. The root is required to query the page, map it in its addre=
ss
space and wrmsr to enable it. It cannot change the location of the page. Fo=
r
guest, it can allocate and assign the hypercall page. This is covered a bit=
 in the
hypervisor TLFS (section 3.13 in TLFS v6), for the guest side. The root sid=
e is=20
not covered there, yet.
=20