On Wed, Jun 03, 2020 at 04:23:25PM +0200, Joerg Roedel wrote: > > > + */ > > > + io_bytes = (exit_info_1 >> 4) & 0x7; > > > + ghcb_count = sizeof(ghcb->shared_buffer) / io_bytes; > > > + > > > + op_count = (exit_info_1 & IOIO_REP) ? regs->cx : 1; > > > + exit_info_2 = min(op_count, ghcb_count); > > > + exit_bytes = exit_info_2 * io_bytes; > > > + > > > + es_base = insn_get_seg_base(ctxt->regs, INAT_SEG_REG_ES); > > > + > > > + if (!(exit_info_1 & IOIO_TYPE_IN)) { > > > + ret = vc_insn_string_read(ctxt, > > > + (void *)(es_base + regs->si), > > > > SEV(-ES) is 64-bit only, why bother with the es_base charade? > > User-space can also cause IOIO #VC exceptions, and user-space can be > 32-bit legacy code with segments, so es_base has to be taken into > account.
Is there actually a use case for this? Exposing port IO to userspace doesn't exactly improve security. Given that i386 ABI requires EFLAGS.DF=0 upon function entry/exit, i.e. is the de facto default, the DF bug implies this hasn't been tested. And I don't see how this could possibly have worked for SEV given that the kernel unrolls string I/O because the VMM can't emulate string I/O. Presumably someone would have complained if they "needed" to run legacy crud. The host and guest obviously need major updates, so supporting e.g. DPDK with legacy virtio seems rather silly. > > > + ghcb->shared_buffer, io_bytes, > > > + exit_info_2, df); > > > > df handling is busted, it's aways non-zero. Same goes for the SI/DI > > adjustments below. > > Right, this is fixed now. > > > Batching the memory accesses and I/O accesses separately is technically > > wrong, e.g. a #DB on a memory access will result in bogus data being shown > > in the debugger. In practice it seems unlikely to matter, but I'm curious > > as to why string I/O is supported in the first place. I didn't think there > > was that much string I/O in the kernel? > > True, #DBs won't be correct anymore. Currently debugging is not > supported in SEV-ES guests anyway, but if it is supported the #DB > exception would happen in the #VC handler and not on the original > instruction. As in, the guest can't debug itself? Or the host can't debug the guest?