RE: [RFC PATCH] x86, entry: Switch stacks on a paranoid entry from userspace

[Luck, Tony]

> v2's not going to make a difference unless you're using uprobes at the
> same time.

Not (knowingly) using uprobes. System is installed with a RHEL7 userspace ... but is essentially
idle except for my test program.

> In the interest of my sanity, can you add something like
> BUG_ON(!user_mode_vm(regs)) or the mce_panic equivalent before calling
> memory_failure?

I don't think that can possibly trip - we can only end up with a recoverable error from
a user mode access.  But I'll see about adding it anyway

> What happens if there's a shared bank but the actual offender has a
> higher order than the cpu that finds the error?

This test case injects a memory error which is logged in bank1. This bank is shared by the
two hyperthreads that are on the same core.  The mce_severity() function distinguishes
which is the active thread and which the innocent bystander by looking at MCG_STATUS.
In the active thread MCG_STATUS.EIPV is 1, in the bystander it is 0. The returned severity
is MCE_AR_SEVERITY for the thread that hit the error, MCE_KEEP_SEVERITY for the bystander.
So it doesn't matter which thread has the lower order and sees it first.

> Is this something I can try under KVM?

I don't know if KVM has a way to simulate a machine check event.

-Tony