James Molloy's Tutorial Known Bugs: Difference between revisions
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== Problem: kmalloc isn't properly aligned == |
== Problem: kmalloc isn't properly aligned == |
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Each data type in C has its own natural alignment. For instance, on the ABI that you are using an int is a signed 32-bit value that must be 32-bit aligned in memory (4 byte alignment). The same applies for structures, where the alignment of the whole structure is the maximum alignment of all its members. It is undefined behavior to access an unaligned value. For instance, you could decide you want an int at a particular unaligned (for an int) memory address and construct a pointer to it. When you attempt to write an int value to that pointer, undefined behavior happens. |
Each data type in C has its own natural alignment. For instance, on the ABI that you are using an int is a signed 32-bit value that must be 32-bit aligned in memory (4 byte alignment). The same applies for structures, where the alignment of the whole structure is the maximum alignment of all its members. It is undefined behavior to access an unaligned value. For instance, you could decide you want an int at a particular unaligned (for an int) memory address and construct a pointer to it. When you attempt to write an int value to that pointer, undefined behavior happens. Furthermore, SIMD registers have alignment needs that are bigger than their individual components. |
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The <tt>kmalloc</tt> function in 6.4.1 only 1-byte aligns or page-aligns its memory address. This means you can only reliably use it allocate memories for chars (size 1), but not any larger types unless you use page-alignment. A proper malloc implementation returns pointers that are aligned such that they are suitable for all the common types, for instance it could be 64-bit (8-byte) aligned. You'll also want to modify the parameters such that it uses <tt>size_t</tt> appropriately rather than <tt>u32int</tt>. |
The <tt>kmalloc</tt> function in 6.4.1 only 1-byte aligns or page-aligns its memory address. This means you can only reliably use it allocate memories for chars (size 1), but not any larger types unless you use page-alignment. A proper malloc implementation returns pointers that are aligned such that they are suitable for all the common types, for instance it could be 64-bit (8-byte) aligned. You'll also want to modify the parameters such that it uses <tt>size_t</tt> appropriately rather than <tt>u32int</tt>. |
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