RE: [PATCH hyperv-next v6 02/11] arm64: hyperv: Use SMCCC to detect hypervisor presence

[Michael Kelley]

From: Mark Rutland <mark.rutland@xxxxxxx> Sent: Monday, March 17, 2025 4:38 AM
> 
> On Fri, Mar 14, 2025 at 05:19:22PM -0700, Roman Kisel wrote:
> > The arm64 Hyper-V startup path relies on ACPI to detect
> > running under a Hyper-V compatible hypervisor. That
> > doesn't work on non-ACPI systems.
> >
> > Hoist the ACPI detection logic into a separate function. Then
> > use the vendor-specific hypervisor service call (implemented
> > recently in Hyper-V) via SMCCC in the non-ACPI case.
> >
> > Signed-off-by: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > ---
> >  arch/arm64/hyperv/mshyperv.c | 43 +++++++++++++++++++++++++++++++-----
> >  1 file changed, 38 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
> > index 2265ea5ce5ad..c5b03d3af7c5 100644
> > --- a/arch/arm64/hyperv/mshyperv.c
> > +++ b/arch/arm64/hyperv/mshyperv.c
> > @@ -27,6 +27,41 @@ int hv_get_hypervisor_version(union
> hv_hypervisor_version_info *info)
> >  	return 0;
> >  }
> >
> > +static bool __init hyperv_detect_via_acpi(void)
> > +{
> > +	if (acpi_disabled)
> > +		return false;
> > +#if IS_ENABLED(CONFIG_ACPI)
> > +	/*
> > +	 * Hypervisor ID is only available in ACPI v6+, and the
> > +	 * structure layout was extended in v6 to accommodate that
> > +	 * new field.
> > +	 *
> > +	 * At the very minimum, this check makes sure not to read
> > +	 * past the FADT structure.
> > +	 *
> > +	 * It is also needed to catch running in some unknown
> > +	 * non-Hyper-V environment that has ACPI 5.x or less.
> > +	 * In such a case, it can't be Hyper-V.
> > +	 */
> > +	if (acpi_gbl_FADT.header.revision < 6)
> > +		return false;
> > +	return strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8) == 0;
> > +#else
> > +	return false;
> > +#endif
> > +}
> > +
> 
> The 'acpi_disabled' variable doesn't exist for !CONFIG_ACPI, so its use
> prior to the ifdeffery looks misplaced.

FWIW, include/linux/acpi.h has 

#define acpi_disabled 1

when !CONFIG_ACPI.  But I agree that using a stub is better.

Michael

> 
> Usual codestyle is to avoid ifdeffery if possible, using IS_ENABLED().
> Otherwise, use a stub, e.g.
> 
> | #ifdef CONFIG_ACPI
> | static bool __init hyperv_detect_via_acpi(void)
> | {
> | 	if (acpi_disabled)
> | 		return false;
> |
> | 	if (acpi_gbl_FADT.header.revision < 6)
> | 		return false;
> |
> | 	return strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8) == 0;
> | }
> | #else
> | static inline bool hyperv_detect_via_acpi(void) { return false; }
> | #endif
> 
> Mark.
> 
> > +static bool __init hyperv_detect_via_smccc(void)
> > +{
> > +	uuid_t hyperv_uuid = UUID_INIT(
> > +		0x4d32ba58, 0x4764, 0xcd24,
> > +		0x75, 0x6c, 0xef, 0x8e,
> > +		0x24, 0x70, 0x59, 0x16);
> > +
> > +	return arm_smccc_hyp_present(&hyperv_uuid);
> > +}
> > +
> >  static int __init hyperv_init(void)
> >  {
> >  	struct hv_get_vp_registers_output	result;
> > @@ -35,13 +70,11 @@ static int __init hyperv_init(void)
> >
> >  	/*
> >  	 * Allow for a kernel built with CONFIG_HYPERV to be running in
> > -	 * a non-Hyper-V environment, including on DT instead of ACPI.
> > +	 * a non-Hyper-V environment.
> > +	 *
> >  	 * In such cases, do nothing and return success.
> >  	 */
> > -	if (acpi_disabled)
> > -		return 0;
> > -
> > -	if (strncmp((char *)&acpi_gbl_FADT.hypervisor_id, "MsHyperV", 8))
> > +	if (!hyperv_detect_via_acpi() && !hyperv_detect_via_smccc())
> >  		return 0;
> >
> >  	/* Setup the guest ID */
> > --
> > 2.43.0
> >