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

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Date:   Sun, 15 Nov 2020 17:13:55 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Andy Lutomirski <luto@...capital.net>
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 15.11.20 17:05, Andy Lutomirski wrote:
> 
>> 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 :)

The PV interface requires to use the iret hypercall, because there
is no sysret equivalent.


Juergen

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