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Subject: Re: WARNING: can't access registers at asm_common_interrupt
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
        linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
        X86 ML <x86@kernel.org>
References: <01c2234c-39a3-2227-25b9-854144c6eb27@suse.com>
 <53DCD372-A847-4B74-A034-330E372B4EB5@amacapital.net>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <65373524-6c5f-cd55-9146-cf3e947bd423@suse.com>
Date:   Sun, 15 Nov 2020 17:13:55 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
 Andrew Cooper <andrew.cooper3@citrix.com>,
 Josh Poimboeuf <jpoimboe@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
 Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>,
 linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
 Damien Le Moal <Damien.LeMoal@wdc.com>, X86 ML <x86@kernel.org>
Message-ID: <65373524-6c5f-cd55-9146-cf3e947bd423@suse.com>
Subject: Re: WARNING: can't access registers at asm_common_interrupt
References: <01c2234c-39a3-2227-25b9-854144c6eb27@suse.com>
 <53DCD372-A847-4B74-A034-330E372B4EB5@amacapital.net>
In-Reply-To: <53DCD372-A847-4B74-A034-330E372B4EB5@amacapital.net>

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On 15.11.20 17:05, Andy Lutomirski wrote:
>=20
>> On Nov 14, 2020, at 10:33 PM, J=C3=BCrgen Gro=C3=9F <jgross@suse.com> =
wrote:
>>
>> =EF=BB=BFOn 14.11.20 19:10, Andy Lutomirski wrote:
>>>> On Sat, Nov 14, 2020 at 1:16 AM J=C3=BCrgen Gro=C3=9F <jgross@suse.c=
om> wrote:
>>>>
>>>> On 13.11.20 18:34, Andy Lutomirski wrote:
>>>>> On Wed, Nov 11, 2020 at 12:25 PM Andrew Cooper
>>>>> <andrew.cooper3@citrix.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_sysr=
et64
>>>> 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 th=
e
>>> 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 retur=
n
>> to user mode via IRET. I think it might work just to use
>> swapgs_restore_regs_and_return_to_usermode() unconditionally for Xen P=
V.
>>
>=20
> I=E2=80=99m 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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