Messages in this thread | | | Date | Tue, 6 Mar 2018 13:27:01 +0100 | From | Joerg Roedel <> | Subject | Re: [PATCH 11/34] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack |
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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
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