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Syscall/ISR acceleration
Author: Jonathan Brandmeyer Date: 2017-01-22 23:22
There are a few features of ARM Cortex-M (the microcontroller profile) that I miss when writing system-level programs in other architectures, including Cortex-A/R:

- Lack of a control register space. In Cortex-M, the processor's control registers are all placed in a per-core region of memory at a reserved physical address. Even the memory protection unit's control registers are saved up there. You don't have to muck around with coprocessor control instructions to manage the caches, memory protection, etc: Its all as simple as addressing into the members of a struct based at some fixed address. I believe, but cannot prove, that this makes visualization easier, since the next level down in the supervisor hierarchy can just map the mocked virtual peripheral to the reserved hardware address.

- Automatic stacking of registers. A major design change in Cortex-M versus ARM7** was that instead of having banked registers for handler mode (a system mode equivalent), the core automatically saved the call-clobbered registers to the stack of the thread mode (user mode equiv) stack pointer. It also saves the link register to a value which points into the system reserved memory space that has the meaning of "return from interrupt". Obvious advantage #1: interrupt handlers can be written in plain ordinary C. Slightly less obvious advantage #2 is that if the processor proceeds to execute another interrupt immediately after the first (say, because the first triggered an OS scheduling event, for example), the user mode stack pointer doesn't need to be saved. Similarly, when performing a context switch between different user-mode threads, only the OS scheduling interrupt must save the rest of the user-mode's thread to the their stack frame. Obvious disadvantage is that the ABI is partially baked into the hardware with respect to the call-clobbered registers and stack pointer.

On a SkyLake-class OoO machine, I suspect that you can execute loads to call-clobbered registers in parallel with the writes of those registers, too, as the ISR goes through the dispatch process.

- Interrupt vectoring. Each peripheral interrupt source gets its own handler instruction assigned to it. This makes interrupt dispatch to the particular driver especially easy. I think this can be taken one step further, where the handler can get an argument or few passed to it. This makes the signature of the interrupt handler something similar to that of a closure in c: "void specific_isr(void *os_provided_data, int peripheral_provided_data)" So long as the function address + arguments are less than the size of cache line, I think it shouldn't cost too much even on a big OoO machine. The interrupt controller can be 'far' away from the core it is dispatching to, passing a message the size of a cache line or less to trigger the interrupt. Now we are also baking in a few argument-passing registers, too.

My professional experience is mostly on microcontroller-class hardware, and having interrupt dispatch as fast as a function call is rather pampering. It seems from the outside that the generally poor performance of interrupt handling on larger processors has lead to a series of design attempts to avoid it, some going as far as polling (!) the device instead of taking an interrupt at the time of demand. My general assumption has been that the cost mostly comes from blowing the dcache and icache for even trivial interrupt responses just due to the branchy/pointer-chasey dispatch logic itself. Just having the peripheral interrupt controller perform the vectoring for the operating system would drastically reduce the data-cache pressure incurred by forcing the ISR to perform all of the pointer-chasing dispatch logic on its own, if that underlying assumption is true.


** ARM7 aka ARM7TDMI is actually ARMv4, while Cortex-M is ARMv7. Except for Cortex-M2x which is ARMv8. Sigh.

 
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