Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack

[Andy Lutomirski]

On Wed, Jan 17, 2018 at 6:10 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> On Wed, Jan 17, 2018 at 05:57:53AM -0800, Brian Gerst wrote:
>> On Wed, Jan 17, 2018 at 1:24 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
>
>> > I have no real idea on how to switch back to the entry stack without
>> > access to per_cpu variables. I also can't access the cpu_entry_area for
>> > the cpu yet, because for that we need to be on the entry stack already.
>>
>> Switch to the trampoline stack before loading user segments.
>
> That requires to copy most of pt_regs from task- to trampoline-stack,
> not sure if that is faster than temporily restoring kernel %fs.
>

I would optimize for simplicity, not speed.  You're already planning
to write to CR3, which is serializing, blows away the TLB, *and* takes
the absurdly large amount of time that the microcode needs to blow
away the TLB.

(For whatever reason, Intel doesn't seem to have hardware that can
quickly wipe the TLB.  I suspect that the actual implementation does
it in a loop and wipes little pieces at a time.  Whatever it actually
does, the CR3 write itself is very slow.)