Support for Skylake X found in High Sierra DP-X

You may remember my blog about XCPM for unsupported Processors and I thought about having another look. This time I looked at the Developer Previews of High Sierra and this is what I found. Let’s start with the switch table of _xcpm_bootstrap:

Ok. Let’s do the math:

0xffffffff - (0xfffffe5a - 0x01) = 0x1a6	// case 0  (0x3c - 0x3c = 0x00/0)
0xffffff80003a6334 - 0x1a6 = 0xffffff80003a618e

0xffffffff - (0xfffffedc - 0x01) = 0x124	// case 1  (0x3d - 0x3c = 0x01/1)
0xffffff80003a6334 - 0x124 = 0xffffff80003a6210

0xffffffff - (0xfffffef5 - 0x01) = 0x10b	// case 9  (0x45 - 0x3c = 0x09/9)
0xffffff80003a6334 - 0x10b = 0xffffff80003a6229

0xffffffff - (0xffffff0b - 0x01) = 0xf5		// case 10 (0x46 - 0x3c = 0x0a/10)
0xffffff80003a6334 - 0xf5 = 0xffffff80003a623f

0xffffffff - (0xffffff2c - 0x01) = 0xd4		// case 11 (0x47 - 0x3c = 0x0b/11)
0xffffff80003a6334 - 0xd4 = 0xffffff80003a6260

0xffffffff - (0xfffffed0 - 0x01) = 0x130	// case 18 (0x4e - 0x3c = 0x12/18)
0xffffff80003a6334 - 0x130 = 0xffffff80003a6204

0xffffffff - (0xffffff38 - 0x01) = 0xc8		// case 25 (0x55 - 0x3c = 0x19/25)
0xffffff80003a6334 - 0xc8 = 0xffffff80003a626c

0xffffffff - (0xfffffec1 - 0x01) = 0x13f	// case 33 (0x5d - 0x3c = 0x21/33)
0xffffff80003a6334 - 0x13f = 0xffffff80003a61f5

0xffffffff - (0xfffffe73 - 0x01) = 0x18d	// case 34 (0x5e - 0x3c = 0x22/34)
0xffffff80003a6334 - 0x18d = 0xffffff80003a61a7

Now we have the offsets and type of processors. And they all match. You can check that yourself below – this is a code snippet of _xcpm_bootstrap in the High Sierra kernel:

ffffff80003a6173	movl	%ebx, %eax
ffffff80003a6175	addb	$-0x3c, %al
ffffff80003a6177	cmpb	$0x22, %al
ffffff80003a6179	ja   	0xffffff80003a619d

ffffff80003a617b	movzbl	%al, %eax
ffffff80003a617e	leaq	0x1af(%rip), %rcx
ffffff80003a6185	movslq	(%rcx,%rax,4), %rax
ffffff80003a6189	addq	%rcx, %rax
ffffff80003a618c	jmpq 	*%rax

ffffff80003a618e	movl	$0x4, _xcpm_hwp_idle_epp(%rip)		// case  0: 0x3c = HASWELL
ffffff80003a6198	jmp 	0xffffff80003a6249

ffffff80003a619d	cmpb	$-0x72, %bl  	 	 	 	 	 	// 0x8e = KABYLAKE
ffffff80003a61a0	je   	0xffffff80003a6204

ffffff80003a61a2	cmpb	$-0x62, %bl  	 	 	 	 	 	// 0x9e	= KABYLAKE_DT
ffffff80003a61a5	jne 	0xffffff80003a61f5

ffffff80003a61a7	movq	0x752fa2(%rip), %rdi			 	// case 34: 0x5E - SKYLAKE_DT and KABYLAKE_DT

ffffff80003a61ae	testq	%rdi, %rdi
ffffff80003a61b1	jne  	0xffffff80003a61c8

ffffff80003a61b3	movq	0x729ed6(%rip), %rax
ffffff80003a61ba	movq	0x488(%rax), %rdi
ffffff80003a61c1	movq	%rdi, 0x752f88(%rip)
ffffff80003a61c8	addq	$0x2, %rdi
ffffff80003a61cc	movl	$0x2, %esi
ffffff80003a61d1	callq	0xffffff800037e070
ffffff80003a61d6	orl 	$0x4000, %eax
ffffff80003a61db	movzwl	%ax, %eax
ffffff80003a61de	cmpl	$0x5910, %eax
ffffff80003a61e3	jne 	0xffffff80003a62a1

ffffff80003a61e9	movl	$0x2000, _xcpm_hwp_idle_epp(%rip)
ffffff80003a61f3	jmp 	0xffffff80003a621a

ffffff80003a61f5	movl	$0x0, 0x6f8aa9(%rip)			 	// case 33: exit
ffffff80003a61ff	jmp 	0xffffff80003a6329

ffffff80003a6204	movl	$0x200, _xcpm_hwp_idle_epp(%rip)	// case 18: 0x4e = SKYLAKE and KABYLAKE
ffffff80003a620e	jmp 	0xffffff80003a621a

ffffff80003a6210	movl	$0x80, _xcpm_hwp_idle_epp(%rip)		// case  1: 0x3d = BROADWELL
ffffff80003a621a	movl	$0x1, 0x689cf0(%rip)
ffffff80003a6224	jmp 	0xffffff80003a62b5

ffffff80003a6229	movl	$0x10, _xcpm_hwp_idle_epp(%rip)		// case  9: 0x45 = HASWELL_ULT
ffffff80003a6233	movl	$0x1, 0x689cd7(%rip)
ffffff80003a623d	jmp 	0xffffff80003a6249

ffffff80003a623f	movl	$0x8, _xcpm_hwp_idle_epp(%rip)		// case 10: 0x46 = CRYSTALWELL
ffffff80003a6249	movl	$0x1, 0x689cc9(%rip)
ffffff80003a6253	movq	$0x0, 0x689d32(%rip)
ffffff80003a625e	jmp 	0xffffff80003a62b5

ffffff80003a6260	movl	$0x40, _xcpm_hwp_idle_epp(%rip)		// case 11: 0x47 = BROADWELL_H
ffffff80003a626a	jmp 	0xffffff80003a62b5

ffffff80003a626c	movl	$0x4000, _xcpm_hwp_idle_epp(%rip)	// case 25: 0x55 = SKYLAKE_X
ffffff80003a6276	movl	$0x0, 0x689c94(%rip)
ffffff80003a6280	movq	$0x989680, 0x689c35(%rip)
ffffff80003a628b	movl	$0x1, 0x689de3(%rip)
ffffff80003a6295	movl	$0x1, _xcpm_ubpc_enabled(%rip)
ffffff80003a629f	jmp 	0xffffff80003a62b5

ffffff80003a62a1	movl	$0x1000, _xcpm_hwp_idle_epp(%rip)
ffffff80003a62ab	movl	$0x0, 0x689c5f(%rip)
ffffff80003a62b5	incl	_xcpm_bootstrap_count(%rip)
ffffff80003a62bb	leaq	0x5d1fa1(%rip), %rdi ## literal pool for: "-xcpm"
ffffff80003a62c2	leaq	-0x14(%rbp), %rsi
ffffff80003a62c6	movl	$0x4, %edx
ffffff80003a62cb	xorl	%ecx, %ecx
ffffff80003a62cd	callq	0xffffff80008ff2c0
ffffff80003a62d2	movl	$0x1, 0x6f89cc(%rip)
ffffff80003a62dc	leaq	0x5d1f86(%rip), %rdi ## literal pool for: "-xcpm_assert"
ffffff80003a62e3	leaq	_xcpm_assert_enable(%rip), %rsi
ffffff80003a62ea	movl	$0x4, %edx
ffffff80003a62ef	xorl	%ecx, %ecx
ffffff80003a62f1	callq	0xffffff80008ff2c0
ffffff80003a62f6	leaq	0x5d1f79(%rip), %rdi ## literal pool for: "-xcpm_assert_trace"
ffffff80003a62fd	leaq	_xcpm_assert_trace(%rip), %rsi
ffffff80003a6304	movl	$0x4, %edx
ffffff80003a6309	xorl	%ecx, %ecx
ffffff80003a630b	callq	0xffffff80008ff2c0
ffffff80003a6310	movl	0x190(%r14), %eax
ffffff80003a6317	cmpl	0x194(%r14), %eax
ffffff80003a631e	sbbl	%eax, %eax
ffffff80003a6320	andl	$0x1, %eax
ffffff80003a6323	movl	%eax, _xcpm_SMT_platform(%rip)
ffffff80003a6329	addq	$0x10, %rsp
ffffff80003a632d	popq	%rbx
ffffff80003a632e	popq	%r14
ffffff80003a6330	popq	%rbp
ffffff80003a6331	retq

The next step is to combine the data and merge it into another table.

Note: If you don’t understand what we do here, then follow the link at the top of this blog article.

Note the new one in the above table. But wait. Let’s make another table. One that shows you what it is:

Well. Wasn’t that easy?

The next time I will do the same for _cpuid_set_info. That one is even more interesting, but I ran out of time…

Edit: It’s nice that the High Sierra kernel supports the Skylake X processor, and it works, but they do lack support for ECC memory, so it isn’t immediately clear why Apple did this. I mean. It cannot, as it stands right now, with the currently available Intel processors, be for the iMac Pro. My best guess is that it will be used for a new / updated Apple product. Presumably also for the yet to be released Xeon processors that will be used in the iMac Pro.

Yes of course. The Kabylake (0x8e) and Kabylake desktop (0x9e) processors are also supported, but Apple isn’t using the switch table for these. They may add it at a later date, but right now, they use simple cmp’s for it. One thing though. There is an additional check for 0x5910 for the Kabylake desktop parts. Note that 0x5912 (one example) is a Kabylake with IGPU.

p.s. I don’t have a hack handy right now (only my MacBook Pro) so I cannot verify if the Skylake X is using 0x5910 or some other processor is using it. Any takers?

Update:
I checked a data sheet of the Intel® Xeon® Processor Scalable family and they also use 0x55 to identify the processor model (CPUID 0x5065x). The only difference is that the stepping ID is either two or four. Not zero.

In other words. Apple could, in theory, use the big Xeon’s after all. Maybe only for the next Mac Pro, but we don’t know that. Only Apple could tell us that, and we all know this, that is never going to happen. That is why I have decided to have a closer look at the used Microcode. That should help.

Update-2: I checked the Intel microcode in the accidentally released firmware and found 0x50652 (stepping B-0) and that is definitely not a Skylake X processor – the B-0 stepping of the Intel® Xeon® Processor Scalable Family is CPUID 0x50652 and the H-0 stepping 0x50654.

See also Xeon Microcode Found In iMac Pro Firmware…

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10 thoughts on “Support for Skylake X found in High Sierra DP-X

  1. haha easy? I’m completely ignorant about this sort of thing, but…
    I have an X299 i7-7900x hack with no kext or kernel patches running, only npci 2000 in boot args and it’s running with full speedstep, all USB ports and HDMI sound. So I guess that supports your findings 🙂

    • Not exactly. I did test the Intel i9-7900X and what you see may not be what you think that it is. Just take a look at the output of AppleIntelInfo.kext and you’ll be surprised.

      Edit: What I was trying to say was that what you are seeing is HWP at work. Not XCPM. For that you need boot argument xcpm=1 with a supported board-id (for the plist) or a patched plist. If you select a board-id that doesn’t set HWP to enable, or not do that from the boot loader, then nothing flies anymore.

  2. Hey piker,

    So if there is the correct jump for 0x55 then do have any guess why I can’t boot with my Skylake-X 7820x? I am stuck at the pluses with no errors if I don’t use a fakecpuid.

    Any idea how to debug this?

    Thank you.

  3. I tried to see what happens in the kernel, but whit no fakecpuid, the kernel is not even loaded (after debugmachkernel, no output), with fakecpuid, there is output, starting from === system boot.

    So what happens before kernel is loaded? boot.efi? can that disable the cpu?

  4. This looks like a toggle to create a special build for imac pro, and only include it in that build. I guess there is no way around this, until this drops down to normal builds.

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