Hi Mike, On Tue, Jul 21, 2020 at 09:01:44PM +0000, Mike Stunes wrote: > I’m running into an MMIO-related bug when I try testing this on our hypervisor. > > During boot, probe_roms (arch/x86/kernel/probe_roms.c) uses > romchecksum over the video ROM and extension ROM regions. In my test > VM, the video ROM romchecksum starts at virtual address > 0xffff8880000c0000 and has length 65536. But, at address > 0xffff8880000c4000, we switch from being video-ROM-backed to being > unbacked by anything. > > With SEV-ES enabled, our platform handles reads and writes to unbacked > memory by treating them as MMIO. So, the read from 0xffff8880000c4000 > causes a #VC, which is handled by do_early_exception. > > In handling the #VC, vc_slow_virt_to_phys fails for that address. My > understanding is that the #VC handler should then add an entry to the > page tables and retry the faulting access. Somehow, that isn’t > happening. From the hypervisor side, it looks like the guest is > looping somehow. (I think the VCPU is mostly unhalted and making > progress, but the guest never gets past that romchecksum.) The guest > never actually makes an MMIO vmgexit for that address. That sounds like your guest is in a page-fault loop, but I can't yet explain why. Can you please find out the instruction which is causing the #VC exception? If a page-fault happens during #VC emulation it is forwared to the page-fault handler, so this should work. But somehow this isn't happening or the page-fault handler can't map the faulting address. Regards, Joerg
