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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Sunil Muthuswamy <sunilmut@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>
Cc:     Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
        "virtualization\@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>,
        Linux Kernel List <linux-kernel@vger.kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Nuno Das Neves <Nuno.Das@microsoft.com>,
        Lillian Grassin-Drake <Lillian.GrassinDrake@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        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
In-Reply-To: <SN4PR2101MB0880AAC1B92038C7FDE3496DC0210@SN4PR2101MB0880.namprd21.prod.outlook.com>
References: <20200914112802.80611-1-wei.liu@kernel.org> <20200914112802.80611-9-wei.liu@kernel.org> <87v9gfjpoi.fsf@vitty.brq.redhat.com> <20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2> <SN4PR2101MB0880AAC1B92038C7FDE3496DC0210@SN4PR2101MB0880.namprd21.prod.outlook.com>
Date:   Thu, 17 Sep 2020 13:06:06 +0200
Message-ID: <87o8m4hdcx.fsf@vitty.brq.redhat.com>
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Sunil Muthuswamy <sunilmut@microsoft.com> writes:

>> 
>> 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?
>> >
>> 
>> My understanding is that we can't, but Sunil can maybe correct me.
>
> That is correct. For root partition, the hypervisor has already allocated the
> hypercall page. The root is required to query the page, map it in its address
> space and wrmsr to enable it. It cannot change the location of the page. For
> 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 side is 
> not covered there, yet.

Ok, so it is guaranteed that root partition doesn't have this page in
its address space yet, otherwise it could've been used for something
else (in case it's just normal memory from its PoV).

Please add a comment about this as it is not really obvious.

Thanks,

-- 
Vitaly