Re: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the VTL mode

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

On 1/8/2025 11:17 AM, Stanislav Kinsburskii wrote:
> On Tue, Jan 07, 2025 at 03:11:15PM -0800, Roman Kisel wrote:

[...]

>>
>> Avoiding using the output hypercall page leads to something like[1]
>> and it looks quite complicated although that's the bare bones, lots
>> of notes.
>>
> 
> How is this related to the original discussion?

I was looking for ways to eliminate what I perceived as the source of
friction in the discussion -- allocating the hypercall output page.

> My concern was about the piece allocating of the output page guarded by
> the VTL config option.>> Thanks,
> Stas
> 
>> [1]
>>
>> /*
>>   * Fast extended hypercall with 20 bytes of input and 16 bytes of
>>   * output for getting a VP register.
>>   *
>>   * NOTES:
>>   *  1. The function is __init only atm, so the XMM context isn't
>>   *     used by the user mode.
>>   *  2. X86_64 only.
>>   *  3. Fast extended hypercalls may use XMM0..XMM6, and XMM is
>>   *     architerctural on X86_64 yet the support should be enabled
>>   *     in the CR's. Here, need RDX, R8 and XMM0 for input and RDX,
>>   *     R8 for output
>>   *  4. No provisions for TDX and SEV-SNP for the sake of simplicity
>>   *     (the hypervisor cannot see the guest registers in the
>>   *     confidential VM), would need to fallback.
>>   *  5. The robust implementation would need to check if fast extended
>>   *     hypercalls are available by checking the synthehtic CPUID leaves.
>>   *     A separate leaf indicates fast output support.
>>   *     It _almost_ certainly has to be, unless somehow disabled, hard
>>   *     to see why that would be needed.
>>   */
>> struct hv_u128 {
>> 	u64 low_part;
>> 	u64 high_part;
>> } __packed;
>>
>> static __init u64 hv_vp_get_register_xfast(u32 reg_name,
>> 		struct hv_u128 *value)
>> {
>> 	u64 control = HV_HYPERCALL_REP_COMP_1 | HVCALL_GET_VP_REGISTERS |
>> 					HV_HYPERCALL_FAST_BIT;
>> 	unsigned long flags;
>> 	u64 hv_status;
>>
>> 	union {
>> 		struct hv_get_vp_registers_input input;
>> 		struct {
>> 			u64 lo;
>> 			u64 hi;
>> 		} __packed as_u128;
>> 	} hv_input;
>> 	register u64 rdx asm("rdx");
>> 	register u64 r8 asm("r8");
>> 	register u64 r12 asm("r12");
>>
>> 	local_irq_save(flags);
>>
>> 	hv_input.as_u128.lo = hv_input.as_u128.hi = 0;
>> 	hv_input.input.header.partitionid = HV_PARTITION_ID_SELF;
>> 	hv_input.input.header.vpindex = HV_VP_INDEX_SELF;
>> 	hv_input.input.header.inputvtl = 0;
>>
>> 	rdx = hv_input.as_u128.lo;
>> 	r8 = hv_input.as_u128.hi;
>> 	r12 = reg_name;
>>
>> 	__asm__ __volatile__(
>> 			"subq		$16, %%rsp\n"
>> 			"movups		%%xmm0, 16(%%rsp)\n"
>> 			"movd		%%r12, %%xmm0\n"
>> 			CALL_NOSPEC
>> 			"movups		16(%%rsp), %%xmm0\n"
>> 			"addq		$16, %%rsp\n"
>> 			: "=a" (hv_status), ASM_CALL_CONSTRAINT,
>> 			"+c" (control), "+r" (rdx), "+r" (r8)
>> 			: THUNK_TARGET(hv_hypercall_pg), "r"(r12)
>> 			: "cc", "memory", "r9", "r10", "r11");
>>
>> 	if (hv_result_success(hv_status)) {
>> 		value->low_part = rdx;
>> 		value->high_part = r8;
>> 	}
>>
>> 	local_irq_restore(flags);
>> 	return hv_status;
>> }
>>
>> #if IS_ENABLED(CONFIG_HYPERV_VTL_MODE)
>> u8 __init get_vtl(void)
>> {
>> 	struct hv_u128 reg_value;
>> 	u64 ret = hv_vp_get_register_xfast(HV_REGISTER_VSM_VP_STATUS, &reg_value);
>>
>> 	if (hv_result_success(ret)) {
>> 		ret = reg_value.low_part & HV_VTL_MASK;
>> 	} else {
>> 		pr_err("Failed to get VTL(error: %lld) exiting...\n", ret);
>> 		BUG();
>> 	}
>>
>> 	return ret;
>> }
>> #endif
>>
>>>
>>> Thanks,
>>> Stas
>>
>> -- 
>> Thank you,
>> Roman
>>

-- 
Thank you,
Roman