[RISC-V][GC] Use 128gb for regions_range on RISCV64

[t-mustafin]

Use 128gb for regions_range cause RISCV64 SV39 memory layout allows to use only 256gb of virtual memory: https://docs.kernel.org/riscv/vm-layout.html#risc-v-linux-kernel-sv39.

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Issue Details

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Use 128gb for regions_range cause RISCV64 SV39 memory layout allows to use only 256gb of virtual memory: https://docs.kernel.org/riscv/vm-layout.html#risc-v-linux-kernel-sv39.

Author:	t-mustafin
Assignees:	-
Labels:	area-GC-coreclr, community-contribution
Milestone:	-

[mangod9]

Nit: might even be easier to make it REGIONS_RESERVATION_SIZE_GB to reduce the 1024 * duplication.

[cshung]

@janvorli, is that 256GB limit something we can deduce from the operating system so that we don't need to hardcode it?

[janvorli]

@cshung unfortunately there is no API to get virtual memory size limit on Linux. However, it seems it would be better to use the value returned by the GCToOSInterface::GetVirtualMemoryLimit derived value as a limit and modify the Unix implementation of the method. Currently it looks like this:

runtime/src/coreclr/gc/unix/gcenv.unix.cpp

Lines 1080 to 1092 in e121d77

	size_t GCToOSInterface::GetVirtualMemoryLimit()
	{
	#ifdef HOST_64BIT
	// There is no API to get the total virtual address space size on
	// Unix, so we use a constant value representing 128TB, which is
	// the approximate size of total user virtual address space on
	// the currently supported Unix systems.
	static const uint64_t _128TB = (1ull << 47);
	return _128TB;
	#else
	return (size_t)-1;
	#endif
	}

@t-mustafin we should also modify the GlobalMemoryStatusEx implementation in PAL here:

runtime/src/coreclr/pal/src/misc/sysinfo.cpp

Lines 473 to 478 in e121d77

	// There is no API to get the total virtual address space size on
	// Unix, so we use a constant value representing 128TB, which is
	// the approximate size of total user virtual address space on
	// the currently supported Unix systems.
	static const UINT64 _128TB = (1ull << 47);
	lpBuffer->ullTotalVirtual = _128TB;

[t-mustafin]

@janvorli GCToOSInterface::GetVirtualMemoryLimit() returns 128TB for non-RISCV archetictures instead of 256GB used by gc_heap::regions_range. To avoid magic constants in code I propose to use half of GCToOSInterface::GetVirtualMemoryLimit() to regions mapping.

And I also added RISCV ifdef for GlobalMemoryStatusEx.

[janvorli]

LGTM, thank you!

[gbalykov]

LGTM