linux-kernel - Re: [PATCH hyperv-next v4 1/6] arm64: hyperv: Use SMCCC to detect hypervisor presence

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Message-Id: <97887849-faa8-429b-862b-daf6faf89481@app.fastmail.com>
Date: Fri, 14 Feb 2025 09:05:50 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Roman Kisel" <romank@...ux.microsoft.com>, bhelgaas@...gle.com,
 "Borislav Petkov" <bp@...en8.de>,
 "Catalin Marinas" <catalin.marinas@....com>,
 "Conor Dooley" <conor+dt@...nel.org>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "Dexuan Cui" <decui@...rosoft.com>,
 "Haiyang Zhang" <haiyangz@...rosoft.com>,
 "H. Peter Anvin" <hpa@...or.com>, krzk+dt@...nel.org,
 Krzysztof Wilczyński <kw@...ux.com>,
 "K. Y. Srinivasan" <kys@...rosoft.com>,
 "Lorenzo Pieralisi" <lpieralisi@...nel.org>,
 "Manivannan Sadhasivam" <manivannan.sadhasivam@...aro.org>,
 "Ingo Molnar" <mingo@...hat.com>, "Rob Herring" <robh@...nel.org>,
 ssengar@...ux.microsoft.com, "Thomas Gleixner" <tglx@...utronix.de>,
 "Wei Liu" <wei.liu@...nel.org>, "Will Deacon" <will@...nel.org>,
 devicetree@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, x86@...nel.org
Cc: benhill@...rosoft.com, bperkins@...rosoft.com, sunilmut@...rosoft.com
Subject: Re: [PATCH hyperv-next v4 1/6] arm64: hyperv: Use SMCCC to detect hypervisor
 presence

On Fri, Feb 14, 2025, at 00:23, Roman Kisel wrote:
> On 2/11/2025 10:54 PM, Arnd Bergmann wrote:

> index a74600d9f2d7..86f75f44895f 100644
> --- a/drivers/firmware/smccc/smccc.c
> +++ b/drivers/firmware/smccc/smccc.c
> @@ -67,6 +67,30 @@ 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)

The interface looks good to me.

> +{
> +	struct arm_smccc_res res = {};
> +	struct {
> +		u32 dwords[4]
> +	} __packed res_uuid;

The structure definition here looks odd because of the
unexplained __packed attribute and the nonstandard byteorder.

The normal uuid_t is defined as an array of 16 bytes,
so if you try to represent it in 32-bit words you need to
decide between __le32 and __be32 representation.

> +	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;
> +
> +	res_uuid.dwords[0] = res.a0;
> +	res_uuid.dwords[1] = res.a1;
> +	res_uuid.dwords[2] = res.a2;
> +	res_uuid.dwords[3] = res.a3;
> +
> +	return uuid_equal((uuid_t *)&res_uuid, hyp_uuid);

The SMCCC standard defines the four words to be little-endian,
so in order to compare them against a uuid byte array, you'd
need to declare the array as __le32 and swap the result
members with cpu_to_le32().

Alternatively you could pass the four u32 values into the
function in place of the uuid.

Since the callers have the same endianess confusion, your
implementation ends up working correctly even on big-endian,
but I find it harder to follow when you call uuid_equal() on
something that is not the actual uuid_t value.

> +
> +#define ARM_SMCCC_HYP_PRESENT(HYP) 								\
> +	({															\
> +		const u32 uuid_as_dwords[4] = {							\
> +			ARM_SMCCC_VENDOR_HYP_UID_ ## HYP ## _REG_0,			\
> +			ARM_SMCCC_VENDOR_HYP_UID_ ## HYP ## _REG_1,			\

I don't think using a macro is helpful here, it just makes
it impossible to grep for ARM_SMCCC_VENDOR_HYP_UID_* values when
reading the source.

I would suggest moving the UUID values into a variable next
to the caller like

#define ARM_SMCCC_VENDOR_HYP_UID_KVM \
    UUID_INIT(0x28b46fb6, 0x2ec5, 0x11e9, 0xa9, 0xca, 0x4b, 0x56, 0x4d, 0x00, 0x3a, 0x74)

and then just pass that into arm_smccc_hyp_present(). (please
double-check the endianess of the definition here, I probably
got it wrong myself).

     Arnd