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VMX: Difference between revisions
→Executing VMXON
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=== Executing VMXON ===
The main entry point for using VMX is through the VMXON instruction. The instruction requires a single operand of a m64 region called the VMXON region. The memory region needs to be 4096-byte aligned (bits 0-11 must be 0) and the only VMCS field that should be modified is the VMCS revision identification field. This ID field should contain the value in bits 0-31 of MSR IA32_VMX_BASIC. In order to prepare a memory address in 32-bit PMode for use as an m64, some modifications need to be made. The upper 32-bits of the m64 on non long mode capable processors have to be 0 or an "invalid memory address" error will occur and a VMEXIT will be called.
<source lang="c">
uint32_t * region = (uint32_t *)allocate_4k_aligned(4096);
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This general process of taking a 32-bit memory address and turning it into a psuedo-64bit int (unsigned long long) will be used for all m64 operands later. VMCLEAR is another example instruction that requires the upper 32-bits of a memory address to be 0.
Long mode capable processors simply requires a 64-bit pointer to the region.
Note: '''The VMXON, VMCLEAR and VMPTRLD instruction must point to the physical address of their respective regions.'''
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