On Wed, Jan 14, 2015 at 8:51 AM, Matt Fleming <matt@xxxxxxxxxxxxxxxxx> wrote: > On Wed, 31 Dec, at 06:37:39PM, Matt Fleming wrote: >> On Wed, 17 Dec, at 08:54:56AM, Andy Lutomirski wrote: >> > > As far as I know, the only way to have continuously functional interrupt >> > > handling across a long mode transition is to install an interrupt vector >> > > table and hope that CPUs actually do something intelligent when >> > > receiving an interrupt with LME=1, LMA=1, and PG=0. Yuck. >> > > >> > > Could we get away with issuing 32-bit EFI calls in compat mode, i.e. >> > > with a 32-bit CPL0 CS but while still in long mode? I think that >> > > delivery of an IST interrupt (which includes both NMI and MCE) will >> > > correctly switch to a fully valid 64-bit state and would correctly >> > > switch back when we execute IRET at the end. (Am I missing some reason >> > > that switching bitness without a privilege level change doesn't work >> > > well? I haven't thought of anything, other than the lack of SS/SP controls >> > > on intra-ring interrupts, but that shouldn't be an issue here.) >> > > >> > > As an added benefit, this would considerably simplify the code. >> >> I can't immediately think of a reason that this wouldn't work, but I've >> Cc'd more x86 folks for additional insight. >> >> I will schedule some time to look into this issue in the new year. >> Thanks Andy. > > I finally got some time to look into this, and running with > __KERNEL32_CS seems to work fine at runtime both with Qemu + 32-bit OVMF > and on my ASUS T100. Manually triggering an MCE exception immediately > before invoking the firmware service recovers gracefully. How are you manually triggering an MCE? I've been playing with some MCE stuff recently, but the only reasonably reliable way I know of to trigger an MCE is using WHEA, and I don't have a box with WHEA, and I assume your ASUS T100 doesn't either. > > Where this won't work so well is at boot time before we jump to the > kernel proper. There, we still need to restore the firmware's GDT so > that interrupts are serviced correctly before ExitBootServices() (in > particular, ia32 Tianocore assumes __KERNEL_CS is a 32-bit CS). Tianocore makes assumptions about the kernel's GDT layout? Yuck. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
