Re: [FYI PATCH 1/3] KVM: nVMX: Don't emulate instructions in guest mode

[Jan Kiszka]

On 29.02.20 21:17, Jim Mattson wrote:
> Since UMIP emulation is broken, I'm not sure why anyone would use it.
> (Sorry, Paolo.)

FWIW, adding "-umip" to the "-cpu host" in my qemu setup works around
the bug.

Jan

> On Sat, Feb 29, 2020 at 11:21 AM Jan Kiszka <jan.kiszka@xxxxxx> wrote:
> > On 29.02.20 20:00, Jim Mattson wrote:
> > > On Sat, Feb 29, 2020 at 10:33 AM Oliver Upton <oupton@xxxxxxxxxx> wrote:
> > > > Hi Jan,
> > > >
> > > > On Sat, Feb 29, 2020 at 10:00 AM Jan Kiszka <jan.kiszka@xxxxxx> wrote:
> > > > > Is this expected to cause regressions on less common workloads?
> > > > > Jailhouse as L1 now fails when Linux as L2 tries to boot a CPU: L2-Linux
> > > > > gets a triple fault on load_current_idt() in start_secondary(). Only
> > > > > bisected so far, didn't debug further.
> > > >
> > > > I'm guessing that Jailhouse doesn't use 'descriptor table exiting', so
> > > > when KVM gets the corresponding exit from L2 the emulation burden is
> > > > on L0. We now refuse the emulation, which kicks a #UD back to L2. I
> > > > can get a patch out quickly to address this case (like the PIO exiting
> > > > one that came in this series) but the eventual solution is to map
> > > > emulator intercept checks into VM-exits + call into the
> > > > nested_vmx_exit_reflected() plumbing.
> > >
> > > If Jailhouse doesn't use descriptor table exiting, why is L0
> > > intercepting descriptor table instructions? Is this just so that L0
> > > can partially emulate UMIP on hardware that doesn't support it?
> >
> > That seems to be the case: My host lacks umip, L1 has it. So, KVM is
> > intercepting descriptor table load instructions to emulate umip.
> > Jailhouse never activates that interception.
> >
> > Jan