Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions | Date | Fri, 24 Jul 2020 00:14:28 +0200 |
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Andy Lutomirski <luto@kernel.org> writes: >> On Jul 23, 2020, at 1:22 PM, Thomas Gleixner <tglx@linutronix.de> wrote: >> Andy Lutomirski <luto@amacapital.net> writes: >>> My suggestion is to enlarge pt_regs. The save and restore logic can >>> probably be in C, but pt_regs is the logical place to put a register >>> that is saved and restored across all entries. >> >> Kinda, but that still sucks because schedule from #PF will get it wrong >> unless you do extra nasties. > > This seems like we’re reinventing the wheel. PKRS is not > fundamentally different from, say, RSP. If we want to save it across > exceptions, we save it on entry and context-switch-out and restore it > on exit and context-switch-in.
It's fundamentally different from RSP because it has state (refcount) attached, which RSP clearly has not. If you get rid of the state then yes.
>>> Whoever does this work will have the delightful job of figuring out >>> whether BPF thinks that the layout of pt_regs is ABI and, if so, >>> fixing the resulting mess. >>> >>> The fact the new fields will go at the beginning of pt_regs will make >>> this an entertaining prospect. >> >> Good luck with all of that. > > We can always cheat like this: > > struct real_pt_regs { > unsigned long pkrs; > struct pt_regs regs; > }; > > and pass a pointer to regs around. What BPF doesn't know about can't hurt it.
Yes, but that's the easy part of the problem :)
Thanks,
tglx
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