cea45cd1bd
This commit allows x86 to completely handle a pending interrupt without switching out to user mode. To handle an interrupt on x86 the APIC *must* generate an exception, prior to you being able to acknowledge it. Previously we only allow exceptions (i.e. interrupts) to be generated outside of kernel mode when we are in user mode. This change allows us to 'poll' for an interrupt and transition the APIC whilst in kernel mode by enabling and taking interrupts at carefully defined points. A pending interrupt will be stored by the exception handler, allowing us to then handle the interrupt and acknowledge the hardware APIC. Handling is done by waiting until after we have 'left' the kernel and are about to switch to user mode and then 'entering' the kernel again by jumping to the interrupt entry point. Handling interrupts entirely in kernel mode provides two advantages * It will allow, in the future, the ability to handle kernel interrupts in situations where we need to handle the interrupt before actually performing the hardware switch back to user mode. This case happens where the user thread is using vt-x and so pending interrupts do not generate an interrupt exception, but rather cause an exception to be generated telling the system that there is a pending interrupt * Where there are multiple pending interrupts it is more efficient to avoid additional switches in and out of the user thread Whilst this change does not enable pre-emption points to handle the interrupt before returning out of `handleSyscall` it should be easily implementable with what is provided. |
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