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60 lines
2.3 KiB
Markdown
60 lines
2.3 KiB
Markdown
Unified Virtual Memory Lite (UVM-lite) From NVVM IR
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===================================================
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This document is for the programming language and compiler
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implementers who target NVVM IR and plan to support Unified Virtual
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Memory Lite (UVM-lite) in their language. It provides the low-level
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details related to supporting kernel launches at the NVVM IR level.
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This document assumes the CUDA runtime is used. For the limits and
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restrictions, please refer to the official CUDA documents.
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Allocating a variable in the unified virtual memory environment.
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----------------------------------------------------------------
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In a system that supports unified virtual memory environment, a
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variable can be allocated at a location where host and other devices
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in the system can reference the variable directly. We call such a
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variable a managed variable and say that the variable has the
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managed attribute or is managed. The attribute can be specified
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using a metadata.
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@xxx = internal addrspace(1) global i32 10, align 4
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...
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!1 = !{i32 addrspace(1)* @xxx, !"managed", i32 1}
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A global variable, e.g., @xxx, can be defined and used as usual, but
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here we have a metadata that specifies the managed attributes. (Note
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that the attribute can only be used with variables in the global
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address space.)
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Accessing a managed variable in the host
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----------------------------------------
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To access a managed variable defined in the NVVM IR code, we should
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retrieve a device pointer first, which can be done using cuModuleGetGlobal().
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CUdeviceptr devp_xxx; // device pointer to xxx
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size_t size_xxx; // size of xxx
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result = cuModuleGetGlobal(&devp_xxx, &size_xxx, hModule, "xxx");
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Whether or not the pointer points to managed memory may be queried
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by calling cuPointerGetAttribute() with the pointer attribute
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CU_POINTER_ATTRIBUTE_IS_MANAGED.
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unsigned int attrVal;
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result = cuPointerGetAttribute(&attrVal, CU_POINTER_ATTRIBUTE_IS_MANAGED, devp_xxx);
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// result will be 1 if the pointer is managed or zero otherwise.
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It is safe to use cuPointerGetAttribute to get the host pointers,
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since the device pointers are opaque.
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void *host_ptr_xxx;
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int *p_xxx;
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result = cuPointerGetAttribute(&host_ptr_xxx, CU_POINTER_ATTRIBUTE_HOST_POINTER, devp_xxx);
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p_xxx = (int *)devp_xxx;
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*p_xxx += 1; // read & write without explicit copying
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