Re: [PATCH 11/34] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack

[Joerg Roedel]

Hi Brian,

On Mon, Mar 05, 2018 at 11:41:01AM -0500, Brian Gerst wrote:
> We can keep the same process as the existing debug/NMI handlers -
> leave the current exception pt_regs on the entry stack and just switch
> to the task stack for the call to the handler.  Then switch back to
> the entry stack and continue.  No copying needed.

I looked into this and things are a bit more complicated than in the NMI
and debug handlers. The current code after pt_regs is set up relies on
%esp pointing to the pt_regs structure. But if pt_regs could be on
another stack we need to carry the pt_regs pointer in another register
through the whole ret_from_exception code-path until we actually switch
back the stack.

Since the code-path is used for all stack/cr3 entry/exit cases we need
to setup the extra pt_regs pointer unconditionally and update all places
that reference it through %esp.

It can certainly be done but it looks like another major surgery in the
entry code to optimize a slow-path for handling unlikely segment-loading
exceptions and debug traps. I am not sure if it's worth it.

Regards,

	Joerg