linux-kernel - Re: WARNING: can't access registers at asm_common

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Date:   Sun, 15 Nov 2020 08:05:55 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        X86 ML <x86@...nel.org>
Subject: Re: WARNING: can't access registers at asm_common_interrupt


> On Nov 14, 2020, at 10:33 PM, Jürgen Groß <jgross@...e.com> wrote:
> 
> On 14.11.20 19:10, Andy Lutomirski wrote:
>>> On Sat, Nov 14, 2020 at 1:16 AM Jürgen Groß <jgross@...e.com> wrote:
>>> 
>>> On 13.11.20 18:34, Andy Lutomirski wrote:
>>>> On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
>>>> <andrew.cooper3@...rix.com> wrote:
>>>> 
>>>> So I think there is at most one of these that wants anything more
>>>> complicated than a plain ALTERNATIVE.  Any volunteers to make it so?
>>>> Juergen, if you do all of them except USERGS_SYSRET64, I hereby
>>>> volunteer to do that one.
>>> 
>>> Why is a plain alternative (either swapgs; sysretq or a jmp xen_sysret64
>>> depending on X86_FEATURE_XENPV) no option?
>>> 
>>> Its not as if this code would run before alternative patching.
>> ALTERNATIVE would "work" in the sense that it would function and be
>> just about as nonsensical as the current code.  Fundamentally, Xen
>> PV's sysret feature is not a drop-in replacement for SYSRET64, and
>> pretending that it is seems unlikely to work well.  I suspect that the
>> current code is some combination of exceedingly slow, non-functional,
>> and incorrect in subtle ways.
>> We should just have a separate Xen PV exit path the same way we have a
>> separate entry path in recent kernels.  *This* is what I'm
>> volunteering to do.
> 
> I don't think there is much work needed. Xen PV does basically a return
> to user mode via IRET. I think it might work just to use
> swapgs_restore_regs_and_return_to_usermode() unconditionally for Xen PV.
> 

I’m quite confident that will work, but I was hoping to get it to work quickly too :)

> 
> Juergen
> <OpenPGP_0xB0DE9DD628BF132F.asc>