RE: [PATCH v11 3/5] arm64: hyperv: Initialize hypervisor on boot

[Michael Kelley <mikelley@microsoft.com>]

From: Mark Rutland <mark.rutland@arm.com> Sent: Monday, August 2, 2021 9:26 AM
> 
> On Tue, Jul 20, 2021 at 07:57:01AM -0700, Michael Kelley wrote:
> > Add ARM64-specific code to initialize the Hyper-V
> > hypervisor when booting as a guest VM.
> >
> > This code is built only when CONFIG_HYPERV is enabled.
> >
> > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> > ---
> >  arch/arm64/hyperv/Makefile   |  2 +-
> >  arch/arm64/hyperv/mshyperv.c | 83 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 84 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/arm64/hyperv/mshyperv.c
> >
> > diff --git a/arch/arm64/hyperv/Makefile b/arch/arm64/hyperv/Makefile
> > index 1697d30..87c31c0 100644
> > --- a/arch/arm64/hyperv/Makefile
> > +++ b/arch/arm64/hyperv/Makefile
> > @@ -1,2 +1,2 @@
> >  # SPDX-License-Identifier: GPL-2.0
> > -obj-y		:= hv_core.o
> > +obj-y		:= hv_core.o mshyperv.o
> > diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> > new file mode 100644
> > index 0000000..2811fd0
> > --- /dev/null
> > +++ b/arch/arm64/hyperv/mshyperv.c
> > @@ -0,0 +1,83 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +/*
> > + * Core routines for interacting with Microsoft's Hyper-V hypervisor,
> > + * including hypervisor initialization.
> > + *
> > + * Copyright (C) 2021, Microsoft, Inc.
> > + *
> > + * Author : Michael Kelley <mikelley@microsoft.com>
> > + */
> > +
> > +#include <linux/types.h>
> > +#include <linux/acpi.h>
> > +#include <linux/export.h>
> > +#include <linux/errno.h>
> > +#include <linux/version.h>
> > +#include <linux/cpuhotplug.h>
> > +#include <asm/mshyperv.h>
> > +
> > +static bool hyperv_initialized;
> > +
> > +static int __init hyperv_init(void)
> > +{
> > +	struct hv_get_vp_registers_output	result;
> > +	u32	a, b, c, d;
> > +	u64	guest_id;
> > +	int	ret;
> 
> As Marc suggests, before looking at the FADT, you need something like:
> 
> 	/*
> 	 * Hyper-V VMs always have ACPI.
> 	 */
> 	if (acpi_disabled)
> 		return 0;
> 
> ... where `acpi_disabled` is defined in <linux/acpi.h> (or via its
> includes), so you don't need to include any additional headers.
> 
> > +
> > +	/*
> > +	 * If we're in a VM on Hyper-V, the ACPI hypervisor_id field will
> > +	 * have the string "MsHyperV".
> > +	 */
> > +	if (strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8))
> > +		return -EINVAL;
> 
> As Marc suggests, it's no an error for a platform to not have Hyper-V,
> so returning 0 in tihs case would be preferable.
> 
> Otherwise this looks fine to me.
> 

Thanks Marc and Mark.  Good point that the code should cleanly
handle the case where a kernel is built with CONFIG_HYPERV but
running somewhere other than as a Hyper-V guest.

Michael