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

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Message-ID: <20250108221918.GA2774@skinsburskii.>
Date: Wed, 8 Jan 2025 14:19:18 -0800
From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
To: Roman Kisel <romank@...ux.microsoft.com>
Cc: hpa@...or.com, kys@...rosoft.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, decui@...rosoft.com,
	eahariha@...ux.microsoft.com, haiyangz@...rosoft.com,
	mingo@...hat.com, mhklinux@...look.com,
	nunodasneves@...ux.microsoft.com, tglx@...utronix.de,
	tiala@...rosoft.com, wei.liu@...nel.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, apais@...rosoft.com, benhill@...rosoft.com,
	ssengar@...rosoft.com, sunilmut@...rosoft.com, vdso@...bites.dev
Subject: Re: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the
 VTL mode

On Wed, Jan 08, 2025 at 12:37:17PM -0800, Roman Kisel wrote:
> 
> 
> 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.
> 

No, output page allocation is the current solution and it is fine.
The source of friction is allocation of this page under config option in
runtime.

Thanks,
Stas

> > 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
>