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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     KY Srinivasan <kys@microsoft.com>
Cc:     "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        "x86\@kernel.org" <x86@kernel.org>,
        "devel\@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Tianyu Lan <Tianyu.Lan@microsoft.com>
Subject: Re: [PATCH] x86/hyper-v: use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
References: <20180615163010.20381-1-vkuznets@redhat.com>
        <SN6PR2101MB11209236A3C18C7CA01F4061DC700@SN6PR2101MB1120.namprd21.prod.outlook.com>
        <MW2PR2101MB111340FC6B6AFA7CEC494498A0700@MW2PR2101MB1113.namprd21.prod.outlook.com>
Date:   Wed, 20 Jun 2018 10:27:21 +0200
In-Reply-To: <MW2PR2101MB111340FC6B6AFA7CEC494498A0700@MW2PR2101MB1113.namprd21.prod.outlook.com>
        (KY Srinivasan's message of "Tue, 19 Jun 2018 18:20:36 +0000")
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KY Srinivasan <kys@microsoft.com> writes:

>> -----Original Message-----
>> From: Michael Kelley (EOSG)
>> Sent: Tuesday, June 19, 2018 10:57 AM
>> To: Vitaly Kuznetsov <vkuznets@redhat.com>; x86@kernel.org
>> Cc: devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; 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>; H. Peter Anvin <hpa@zytor.com>; Tianyu Lan
>> <Tianyu.Lan@microsoft.com>
>> Subject: RE: [PATCH] x86/hyper-v: use cheaper
>> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
>> 
>> ...
>>>
>> This is a good idea.  We should probably do the same with the hypercalls for
>> sending
>> IPIs -- try the simpler version first and move to the more complex _EX
>> version only
>> if necessary.
> I am not sure if this would work correctly. When I was developing the IPI enlightenment, 
> what I remember was that the guest is expected to use the API recommended by the Hypervisor.
>

I was under the same impression when I implemented PV TLB flush. Turns
out HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED is a misnomer or at least
Windows treats it as HV_X64_EX_PROCESSOR_MASKS_AVAILABLE instead using
only when needed.

My guess would be that the situation with IPI is the same. In any case I
can try to implement Hyper-V style PV IPIs for Windows in KVM and we'll
see how Windows uses these hypercalls :-)

-- 
  Vitaly