On Mon, Jun 4, 2018 at 8:11 PM Laszlo Ersek <ler...@redhat.com> wrote: > > Hi! > > Apologies if this isn't the right place for asking. For the problem > statement, I'll simply steal Ard's writeup [1]: > > > KVM on ARM refuses to decode load/store instructions used to perform > > I/O to emulated devices, and instead relies on the exception syndrome > > information to describe the operand register, access size, etc. This > > is only possible for instructions that have a single input/output > > register (as opposed to ones that increment the offset register, or > > load/store pair instructions, etc). Otherwise, QEMU crashes with the > > following error > > > > error: kvm run failed Function not implemented > > [...] > > QEMU: Terminated > > > > and KVM produces a warning such as the following in the kernel log > > > > kvm [17646]: load/store instruction decoding not implemented
This looks like a kvm/qemu issue to me. Whatever that exception syndrome thing is, it surely has a pointer to the offending instruction it could decode? > > > > GCC with LTO enabled will emit such instructions for Mmio[Read|Write] > > invocations performed in a loop, so we need to disable LTO [...] > > We have a Red Hat Bugzilla about the (very likely) same issue [2]. > > Earlier, we had to work around the same on AArch64 too [3]. > > Would it be possible to introduce a dedicated -mXXX option, for ARM and > AArch64, that disabled the generation of such multi-operand > instructions? > > I note there are several similar instructions (for other architectures): > * -mno-multiple (ppc) > * -mno-fused-madd (ia64) > * -mno-mmx and a lot of friends (x86) > > Obviously, if the feature request is deemed justified, we should provide > the exact family of instructions to disable. I'll leave that to others > on the CC list with more ARM/AArch64 expertise; I just wanted to get > this thread started. (Sorry if the option is already being requested > elsewhere; I admit I didn't search the GCC bugzilla.) > > Thanks! > Laszlo > > [1] https://lists.01.org/pipermail/edk2-devel/2018-June/025476.html > [2] https://bugzilla.redhat.com/show_bug.cgi?id=1576593 > [3] https://github.com/tianocore/edk2/commit/2efbf710e27a
