On Sun, 3 Nov 2019 at 04:41, Shawn Landden <1851...@bugs.launchpad.net> wrote: > While qemu's scalar emulation tends to be excellent, qemu's SIMD > emulation tends to be incorrect (except for arm64 from x86_64)--i have > found this both for mipsel and arm32. Until these code paths are > audited, which is probably a large job, it would be nice if qemu knew > its emulation of this class of instructions was not very good, and thus > it would give up on finding these instructions if a "careful" operation > is passed.
I'm not sure how this could work. If QEMU reports (via ID regs etc) to the guest that it supports instruction class X when it does not, that's a bug and we should fix it. If QEMU implements an instruction but gets it wrong, that's also a bug and we should fix it. In both cases, we'd need to have specific bug reports, ideally with reproduce-cases. But we don't really have "known areas where the emulation is incorrect" that we could somehow differentiate and disable (except at a very vague level, eg "probably better not to rely on the x86 emulation"). You might be able by careful selection of the cpu type to avoid CPUs which implement vector operations. Some architectures also allow individual CPU features to be disabled with extra '-foo' qualifiers on the -cpu argument. For Arm in particular (32 or 64 bit) I believe our implementation should be correct and am happy to look at bug reports for that. thanks -- PMM
