Re: [PATCH hyperv-next v6 01/11] arm64: kvm, smccc: Introduce and use API for detecting hypervisor presence

Mark Rutland
2025-03-17T11:29:40Z

On Fri, Mar 14, 2025 at 05:19:21PM -0700, Roman Kisel wrote:
> The KVM/arm64 uses SMCCC to detect hypervisor presence. That code is
> private, and it follows the SMCCC specification. Other existing and
> emerging hypervisor guest implementations can and should use that
> standard approach as well.
>
> Factor out a common infrastructure that the guests can use, update KVM
> to employ the new API. The central notion of the SMCCC method is the
> UUID of the hypervisor, and the API follows that.
>
> No functional changes. Validated with a KVM/arm64 guest.
>
> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> ---
> arch/arm64/kvm/hypercalls.c | 5 +--
> drivers/firmware/smccc/kvm_guest.c | 10 ++----
> drivers/firmware/smccc/smccc.c | 19 +++++++++++
> include/linux/arm-smccc.h | 55 +++++++++++++++++++++++++++---
> 4 files changed, 73 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 27ce4cb44904..92b9bc1ea8e8 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -353,10 +353,7 @@ int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
> val[0] = gpa;
> break;
> case ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID:
> - val[0] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0;
> - val[1] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1;
> - val[2] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2;
> - val[3] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3;
> + UUID_TO_SMCCC_RES(ARM_SMCCC_VENDOR_HYP_UID_KVM, val);
> break;
> case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
> val[0] = smccc_feat->vendor_hyp_bmap;
> diff --git a/drivers/firmware/smccc/kvm_guest.c b/drivers/firmware/smccc/kvm_guest.c
> index f3319be20b36..b5084b309ea0 100644
> --- a/drivers/firmware/smccc/kvm_guest.c
> +++ b/drivers/firmware/smccc/kvm_guest.c
> @@ -14,17 +14,11 @@ static DECLARE_BITMAP(__kvm_arm_hyp_services, ARM_SMCCC_KVM_NUM_FUNCS) __ro_afte
>
> void __init kvm_init_hyp_services(void)
> {
> + uuid_t kvm_uuid = ARM_SMCCC_VENDOR_HYP_UID_KVM;
> struct arm_smccc_res res;
> u32 val[4];
>
> - if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
> - return;
> -
> - arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
> - if (res.a0 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0 ||
> - res.a1 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1 ||
> - res.a2 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2 ||
> - res.a3 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3)
> + if (!arm_smccc_hyp_present(&kvm_uuid))
> return;
>
> memset(&res, 0, sizeof(res));
> diff --git a/drivers/firmware/smccc/smccc.c b/drivers/firmware/smccc/smccc.c
> index a74600d9f2d7..7399f27d58e5 100644
> --- a/drivers/firmware/smccc/smccc.c
> +++ b/drivers/firmware/smccc/smccc.c
> @@ -67,6 +67,25 @@ s32 arm_smccc_get_soc_id_revision(void)
> }
> EXPORT_SYMBOL_GPL(arm_smccc_get_soc_id_revision);
>
> +bool arm_smccc_hyp_present(const uuid_t *hyp_uuid)
> +{
> + struct arm_smccc_res res = {};
> +
> + if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
> + return false;
> + arm_smccc_1_1_hvc(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
> + if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
> + return false;
> +
> + return ({
> + const uuid_t uuid = SMCCC_RES_TO_UUID(res.a0, res.a1, res.a2, res.a3);
> + const bool present = uuid_equal(&uuid, hyp_uuid);
> +
> + present;
> + });
> +}

This use of a statement expression is bizarre, and the function would be
clearer without it, e.g.

| bool arm_smccc_hyp_present(const uuid_t *hyp_uuid)
| {
| struct arm_smccc_res res = {};
| uuid_t uuid;
|
| if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
| return false;
|
| arm_smccc_1_1_hvc(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
| if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
| return false;
|
| uuid_t = SMCCC_RES_TO_UUID(res.a0, res.a1, res.a2, res.a3);
| return uuid_equal(&uuid, hyp_uuid);
| }

As noted below, I'd prefer if this were renamed to something like
arm_smccc_hypervisor_has_uuid(), to more clearly indicate what is being
checked.

[...]

> +/**
> + * arm_smccc_hyp_present(const uuid_t *hyp_uuid)
> + *
> + * Returns `true` if the hypervisor advertises its presence via SMCCC.
> + *
> + * When the function returns `false`, the caller shall not assume that
> + * there is no hypervisor running. Instead, the caller must fall back to
> + * other approaches if any are available.
> + */
> +bool arm_smccc_hyp_present(const uuid_t *hyp_uuid);

I'd prefer if this were:

| /*
| * Returns whether a specific hypervisor UUID is advertised for the
| * Vendor Specific Hypervisor Service range.
| */
| bool arm_smccc_hypervisor_has_uuid(const uuid_t *uuid);

> +#define SMCCC_RES_TO_UUID(r0, r1, r2, r3) \
> + UUID_INIT( \
> + cpu_to_le32(lower_32_bits(r0)), \
> + cpu_to_le32(lower_32_bits(r1)) & 0xffff, \
> + cpu_to_le32(lower_32_bits(r1)) >> 16, \
> + cpu_to_le32(lower_32_bits(r2)) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r2)) >> 8) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r2)) >> 16) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r2)) >> 24) & 0xff, \
> + cpu_to_le32(lower_32_bits(r3)) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r3)) >> 8) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r3)) >> 16) & 0xff, \
> + (cpu_to_le32(lower_32_bits(r3)) >> 24) & 0xff \
> + )

I think this'd be clearer if we did something similar to what we did for
the SMCCC SOC_ID name:

https://lore.kernel.org/linux-arm-kernel/20250219005932.3466-1-paul@os.amperecomputing.com/

... and pack/unpack the bytes explicitly, e.g.

| static inline uuid smccc_res_to_uuid(u32 r0, u32, r1, u32 r2, u32 r3)
| {
| uuid_t uuid = {
| .b = {
| [0] = (r0 >> 0) & 0xff,
| [1] = (r0 >> 8) & 0xff,
| [2] = (r0 >> 16) & 0xff,
| [3] = (r0 >> 24) & 0xff,
|
| [4] = (r1 >> 0) & 0xff,
| [5] = (r1 >> 8) & 0xff,
| [6] = (r1 >> 16) & 0xff,
| [7] = (r1 >> 24) & 0xff,
|
| [8] = (r2 >> 0) & 0xff,
| [9] = (r2 >> 8) & 0xff,
| [10] = (r2 >> 16) & 0xff,
| [11] = (r2 >> 24) & 0xff,
|
| [12] = (r3 >> 0) & 0xff,
| [13] = (r3 >> 8) & 0xff,
| [14] = (r3 >> 16) & 0xff,
| [15] = (r3 >> 24) & 0xff,
| },
| };
|
| return uuid;
| }

... which is a bit more verbose, but clearly aligns with what the SMCCC
spec says w.r.t. packing/unpacking, and should avoid warnings about
endianness conversions.

> +
> +#define UUID_TO_SMCCC_RES(uuid_init, regs) do { \
> + const uuid_t uuid = uuid_init; \
> + (regs)[0] = le32_to_cpu((u32)uuid.b[0] | (uuid.b[1] << 8) | \
> + ((uuid.b[2]) << 16) | ((uuid.b[3]) << 24)); \
> + (regs)[1] = le32_to_cpu((u32)uuid.b[4] | (uuid.b[5] << 8) | \
> + ((uuid.b[6]) << 16) | ((uuid.b[7]) << 24)); \
> + (regs)[2] = le32_to_cpu((u32)uuid.b[8] | (uuid.b[9] << 8) | \
> + ((uuid.b[10]) << 16) | ((uuid.b[11]) << 24)); \
> + (regs)[3] = le32_to_cpu((u32)uuid.b[12] | (uuid.b[13] << 8) | \
> + ((uuid.b[14]) << 16) | ((uuid.b[15]) << 24)); \
> + } while (0)
> +
> +#endif /* !__ASSEMBLER__ */

IMO it'd be clearer to initialise a uuid_t beforehand, and then allow
the helper to unpack the bytes, e.g.

static inline u32 smccc_uuid_to_reg(const uuid_t uuid, int reg)
{
u32 val = 0;

val |= (u32)(uuid.b[4 * reg + 0] << 0)
val |= (u32)(uuid.b[4 * reg + 1] << 8)
val |= (u32)(uuid.b[4 * reg + 2] << 16)
val |= (u32)(uuid.b[4 * reg + 3] << 24)

return val:
}

#define UUID_TO_SMCCC_RES(uuid, regs) \
do { \
(regs)[0] = smccc_uuid_to_reg(uuid, 0); \
(regs)[1] = smccc_uuid_to_reg(uuid, 1); \
(regs)[2] = smccc_uuid_to_reg(uuid, 2); \
(regs)[3] = smccc_uuid_to_reg(uuid, 3); \
} while (0)

... though arguably at that point you can get rid of the
UUID_TO_SMCCC_RES() macro and just expand that directly at the callsite.

Mark.