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by amluto
2383 days ago
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I suspect that, at least for lower-power or lower-area designs, a compact ISA with a friendlier encoding than x86 would be a win. A significant problem with x86 is that a high-performance core needs to decode multiple instructions per cycle, but x86 has a nasty problem that the length of an instruction can’t be determined until it’s fully decoded. I think that modern front-ends try all possible offsets at once and throw out the wrong guesses. This costs area and power. A design where all instructions have one of just a few sizes and where the first byte unambiguously encodes the length would be nicer. RISV-V is decent in this respect. FWIW, x86’s legacy is a security problem, too. The ISA is so overcomplicated that nasty interactions cause all manner of security bugs. As a recent example, the sequence mov (ptr), %ss; syscall with a data breakpoint at ptr could be used to root most kernels. With virtualization, this type of thing is much worse. A hypervisor needs to handle all the nasty corner cases in a guest user program without crashing itself or the guest kernel, and it needs to handle all the nasty corner cases in guest kernels without dying. There are various ways that native kernels can literally put the microcode in an infinite loop, and hypervisors need complex mitigations because an infinite-looping microcode bug triggered by a guest can’t be preempted by the host, and it will take down the system. So yes, x86 is not fantastic. |
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