linux-kernel - RE: [PATCH 2/4] arm64: hyperv: Add support for Hyper-V as a hypervisor

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Message-ID: <CY4PR21MB07733F173EDDEA6B83458EFED79F0@CY4PR21MB0773.namprd21.prod.outlook.com>
Date:   Mon, 21 Jan 2019 04:38:44 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        Marc Zyngier <marc.zyngier@....com>,
        KY Srinivasan <kys@...rosoft.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "apw@...onical.com" <apw@...onical.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        vkuznets <vkuznets@...hat.com>
Subject: RE: [PATCH 2/4] arm64: hyperv: Add support for Hyper-V as a
 hypervisor

From: Michael Kelley  Sent: Friday, January 4, 2019 12:05 PM
> 
> > >> As Will said, this isn't a viable option. Please follow SMCCC 1.1.
> > >
> > > I'll have to start a conversation with the Hyper-V team about this.
> > > I don't know why they chose to use HVC #1 or this register scheme
> > > for output values.  It may be tough to change at this point because
> > > there are Windows guests on Hyper-V for ARM64 that are already
> > > using this approach.
> >
> > I appreciate you already have stuff in the wild, but there is definitely
> > a case to be made for supporting architecturally specified mechanisms in
> > a hypervisor, and SMCCC is definitely part of it (I'm certainly curious
> > of how you support the Spectre mitigation otherwise).
> >
> 
> The Hyper-V guys I need to discuss this with are not back from the
> holidays until January 7th.  I'll follow up on this thread once I've
> had that conversation.
> 

Feedback from the Hyper-V guys is that they believe the Hyper-V
specific hypercall sequence *is* compliant with SMCCC 1.1, in the
sense of being outside the requirements per Section 2.9 since the
Hyper-V hypercall sequence uses HVC #1.  Hyper-V wanted to use a
simpler calling sequence that doesn't have the full register
save/restore requirements, for better performance.  The details
of the Hyper-V hypercall sequence are documented internally,
but we do still need to get it published externally as part of a
Hyper-V TLFS version that includes ARM64.

Hyper-V uses the full SMC Calling Conventions in other places,
such as for PSCI calls, for SMCs to EL3, and for the Spectre
mitigation related calls.

Michael