go: x/sys/cpu: cpu.X86.HasAVX512 is incorrectly always false on darwin

Before I go to the work of putting together a CL, here is my proposed implementation of the darwin commpage check:

In cpu_gc_x86.go add:

// darwinHasAVX512 is implemented in cpu_x86.s for gc compiler
// and in cpu_gccgo.go for gccgo.
func darwinHasAVX512() bool

In cpu_gccgo_x86.go add:

// gccgo doesn't build on Darwin, per:
// https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/gcc.rb#L7
func darwinHasAVX512() bool {
	return false
}

In cpu_x86.go (note addition of conditional darwin codepath):

if X86.HasOSXSAVE {
	eax, _ := xgetbv()
	// Check if XMM and YMM registers have OS support.
	osSupportsAVX = isSet(1, eax) && isSet(2, eax)

	if runtime.GOOS == "darwin" {
		// Check darwin commpage for AVX512 support
		osSupportsAVX512 = osSupportsAVX && darwinHasAVX512()
	} else {
		// Check if OPMASK and ZMM registers have OS support.
		osSupportsAVX512 = osSupportsAVX && isSet(5, eax) && isSet(6, eax) && isSet(7, eax)
	}
}

In cpu_x86.s add this code:

// func darwinHasAVX512() bool
TEXT ·darwinHasAVX512(SB), NOSPLIT, $0-1

    MOVB    $0, ret+0(FP) // default to false

#ifdef GOOS_darwin   // return if not darwin
#ifdef GOARCH_amd64  // return if not amd64

// These values from:
// https://github.com/apple/darwin-xnu/blob/xnu-4570.1.46/osfmk/i386/cpu_capabilities.h 
#define commpage64_base_address         0x00007fffffe00000
#define commpage64_cpu_capabilities64   (commpage64_base_address+0x010)
#define commpage64_version              (commpage64_base_address+0x01E) 
#define hasAVX512F                      0x0000004000000000

    MOVQ    $commpage64_version, BX
    MOVW    (BX), AX
    CMPW    AX, $13  // versions < 13 do not support AVX512
    JL      no_avx512
    MOVQ    $commpage64_cpu_capabilities64, BX
    MOVQ    (BX), AX
    MOVQ    $hasAVX512F, CX
    ANDQ    CX, AX
    JZ      no_avx512
    MOVB    $1, ret+0(FP) 
no_avx512:

#endif
#endif

    RET

Note that the cpu_capabilities field occurs above the page_version in the commpage layout. Seems very unlikely these fields will ever move or change.

Also note the use of conditional compilation above to render the darwinHasAVX512 function a stub that always returns false in all cases except GOOS=="darwin" && GOARCH=="amd64". This seems preferable to the alternative, which would be to create two new separate assembly files for the only purpose of providing the above function and its stub in those two cases.

Based on my understanding of everything, I think if the commpage advertises kHasAVX512F then it is safe to assume that darwin will promote threads attempting to use AVX512 instructions.

But I think that is different from what you are suggesting, if I’m understanding correctly, which it that if the CPUID bit for AVX512F is set then we can probably safely make that same assumption.

As I say above, I think that is almost certainly true for unmodified Apple hardware running unpatched MacOS.

Outside of that ideal walled garden, all bets are off. So one question is, does golang as a project care about this case? I assume it does, otherwise why call the OS target “darwin” and not “macos” or whatever? But I honestly don’t know the history of this.

Virtualization keeps coming up, and I think it also raises important potential cases. To my knowledge it is certainly plausible that an older (pre-AVX512) version of MacOS could be run on newer hardware that supports AVX512 under virtualization. If in that case the hypervisor chooses to enable AVX512 within that VM, then yes, it would be incorrect for golang to rely exclusively on the CPUID bits to determine support. If this scenario is indeed possible, it is another argument for checking the commpage for kHasAVX512F (which an older version of darwin shouldn’t set, regardless of what the CPUID says). What I don’t know is if the checks MacOS makes under virtualization to ensure it is running on genuine Mac hardware obey the same constraints as when it is running on bare metal.

To be clear, “older” here means MacOS 10.12 Sierra or before, i.e. versions of MacOS/OS X that are no longer supported by Apple.