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    SubjectRe: [PATCH v3 25/75] x86/sev-es: Add support for handling IOIO exceptions
    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?

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