RE: [RFC] x86_64: A real proposal for iret-less return to kernel

[Luck, Tony]

On Wed, May 21, 2014 at 03:39:11PM -0700, Andy Lutomirski wrote:
> But if we get a new MCE in here, it will be an MCE from kernel context
> and it's fatal. So, yes, we'll clobber the stack, but we'll never
> return (unless tolerant is set to something insane), so who cares?

Remember that machine checks are broadcast.  So some other cpu
can hit a recoverable machine check in user mode ... but that int#18
goes everywhere.  Other cpus are innocent bystanders ... they will
see MCG_STATUS.RIPV=1, MCG_STATUS.EIPV=0 and nothing important
in any of their machine check banks.

But if we are still finishing off processing the previous machine check,
this will be a nested one - and BOOM, we are dead.

-Tony

[If you peer closely at the latest edition of the SDM - you'll see the
bits are defined for a non-broadcast model ... e.g. LMCE_S bit in
MCG_STATUS .... but currently shipping silicon doesn't use that]