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pm215 parent
Wait, x86 still requires instruction emulation for non-weirdo non-legacy cases? My vague recollection of the KVM Forum talk G. did was that you don't need it for "modern" guests.

(We were talking about emulation-via-just-interpret-one-instruction in userspace in upstream QEMU the other day -- you'd want it for OSX hypervisor.framework support too, after all. And maybe for the corner cases in TCG where you'd otherwise emulate one instruction and throw away the cached translation immediately.)

bonzini
Apart from the legacy case, you need it for MMIO---KVM for ARM also has a mini parser for LDR/STR instructions.

x86 however has all sorts of wonderful read-modify-write instructions too. You need to support those, but it would still be a small subset of the full x86 instruction set if you all you want to support is processors newer than circa 2010.

pm215 OP
KVM for ARM doesn't parse instructions -- you can just use the register info the hardware gives you in the syndrome register, which covers everything except oddball cases like trying load-multiple to a device, which doesn't happen in practice and so we don't support it.
jsolson
Yeah, it still gets hit now and then. It should not get hit often in the typical steady state, though, which is why you can punt it to userspace with little performance penalty.

(I work on the custom VMM we run)

strstr
(Echoing Bonzini) You don't need it to be in the kernel for modern guests (performance wise), but you still need it.

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