Re: [Xen-devel] [PATCH v2] x86/boot: enable NMIs after traps init

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Tue, Oct 23, 2018 at 10:46 AM Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 23/10/18 15:01, Jason Andryuk wrote:
> > On Tue, Oct 23, 2018 at 7:15 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> > wrote:
> >> On 23/10/18 11:59, Sergey Dyasli wrote:
> >>> In certain scenarios, NMIs might be disabled during Xen boot process.
> >>> Such situation will cause alternative_instructions() to:
> >>>
> >>>     panic("Timed out waiting for alternatives self-NMI to hit");
> >>>
> >>> This bug was originally seen when using Tboot to boot Xen.
> >>>
> >>> To prevent this from happening, enable NMIs during cpu_init() and
> >>> during __start_xen() for BSP.
> >>>
> >>> Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
> >> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > FYI, Ross and Eric came up with a tboot patch recently added to OpenXT:
> > https://github.com/OpenXT/xenclient-oe/blob/master/recipes-openxt/tboot/tboot-1.9.6/0023-tboot-Unmask-NMI-potentially-masked-during-SENTER.patch
> >
> > Using this Xen patch with the tboot one reverted works too.
> >
> > Tested-by: Jason Andryuk <jandryuk@xxxxxxxxx>
>
> :(
>
> Can bugs like this please be reported upstream?  Given the observation
> of "Tboot hands off with NMIs disabled", the fix is very easy.

I'm not opposed to reporting upstream.  In this case, I at least
assumed it was something we did in our EFI & tboot combo.  An Ivy
Bridge legacy boot system with tboot & Xen worked fine.  For me, it
was only a newer Skylake (or Kaby Lake) machine that had issue when
booting our EFI & tboot combo.  So it wasn't clear that tboot always
left NMIs disabled.  Yes, we should have reported something upstream
as a heads up for other tboot/Xen users.

Regards,
Jason

> On the subject of having NMIs disabled, it is definitely a more robust
> way of handing off between components.  Until Xen has transitioned the
> BSP and APs into 64bit mode and fully set the IDT up, NMIs are fatal to
> the system.
>
> Furthermore, I've got some plans to suggest we do this on the kexec path
> as well.  XenServer has one example where we crash from a GPU related
> error, and a slightly delayed NMI arrives, usually when we're in purgatory.
>
> ~Andrew

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