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

Michael Kelley
2025-03-19T22:11:54Z

From: Mark Rutland <mark.rutland@arm.com> 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@linux.microsoft.com>
> > Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> > ---
> > 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
> >