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

[Roman Kisel <romank@linux.microsoft.com>]

On 3/17/2025 4:29 AM, Mark Rutland wrote:
> On Fri, Mar 14, 2025 at 05:19:21PM -0700, Roman Kisel wrote:

[...]

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

I'll change that to what you're suggesting, thanks for your help!

> 
> | 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.
> 
> [...]

Will update, thanks for your help!

> 
>> +/**
>> + * 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);
> 
[...]



> 
> 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.

That looks great, thanks for the suggestion!

> 
> | 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.
> 

I believe what you're proposing hits a better trade-off, thanks again!

>> +
>> +#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.
> 

I'll work on that, thanks!!

> Mark.

-- 
Thank you,
Roman