linux-kernel - Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

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Message-ID: <874kpxx4jf.fsf@nanos.tec.linutronix.de>
Date:   Fri, 24 Jul 2020 00:14:28 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Weiny Ira <ira.weiny@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Andy Lutomirski <luto@...nel.org> writes:
>> On Jul 23, 2020, at 1:22 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> Andy Lutomirski <luto@...capital.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