Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220712183238.844813653@linuxfoundation.org> <CA+G9fYtntg7=zWSs-dm+n_AUr_u0eBOU0zrwWqMeXZ+SF6_bLw@mail.gmail.com>
 <eb63e4ce-843f-c840-060e-6e15defd3c4d@roeck-us.net>
In-Reply-To: <eb63e4ce-843f-c840-060e-6e15defd3c4d@roeck-us.net>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 13 Jul 2022 11:33:22 -0700
Message-ID: <CAHk-=wj5cOA+fbGeV15kvwe6YGT54Wsk8F2UGoekVQLTPJz_pw@mail.gmail.com>
Subject: Re: [PATCH 5.15 00/78] 5.15.55-rc1 review
To:     Guenter Roeck <linux@roeck-us.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@suse.de>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        kvm list <kvm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, patches@kernelci.org,
        lkft-triage@lists.linaro.org, Pavel Machek <pavel@denx.de>,
        Jon Hunter <jonathanh@nvidia.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Slade Watkins <slade@sladewatkins.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>,
        Anders Roxell <anders.roxell@linaro.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Jul 13, 2022 at 6:34 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> Looking into the log, I don't think that message is related to the crash.
>
> ...
> [  105.653777] Modules linked in: x86_pkg_temp_thermal
> [  105.902123] ---[ end trace cec99cae36bcbfd7 ]---
> [  105.902124] RIP: 0010:xaddw_ax_dx+0x9/0x10    <--- crash
> [  105.902126] Code: 00 0f bb d0 c3 cc cc cc cc 48 0f bb d0 c3 cc cc

Yeah, the code you snipped, shows

  20: 66 0f c1 d0          xadd   %dx,%ax
  24: c3                    ret
  25: cc                    int3
  26: cc                    int3
  27: cc                    int3
  28: cc                    int3
  29:* 0f 1f 80 00 00 00 00 nopl   0x0(%rax) <-- trapping instruction
  30: 0f c1 d0              xadd   %edx,%eax
  33: c3                    ret
  34: cc                    int3
  35: cc                    int3
  36: cc                    int3
  37: cc                    int3
  38: 48 0f c1 d0          xadd   %rdx,%rax
  3c: c3                    ret
  3d: cc                    int3

and that's a bit odd.

It says "xaddw_ax_dx+0x9/0x10", but I think somebody jumped to
"xaddw_ax_dx+8", hit the 'int3', and the RIP points to the next
instruction (because that's how int3 works).

And the fastop code says:

 * fastop functions have a special calling convention:
...
 * Moreover, they are all exactly FASTOP_SIZE bytes long,

but that is clearly *NOT* the case for xaddw_ax_dx, because it's 16
bytes in size, and the other ones are 8 bytes. That's where the "nopl"
comes from: it's the alignment instruction to the next fastop
function.

Compare that to the word-sized 'xaddl' case rigth afterwards: that one
*is* just 8 bytes in size, so the 64-byte 'xaddq' comes 8 bytes aftrer
it, and there's no 7-byte padding nop-instruction.

So I think that that is where the "xaddw_ax_dx+8" comes from: some
code assumes that FASTOP_SIZE is 8, but that xaddw_ax_dx case was
actually 9 bytes and thus got that "int3 + padding" in the next 8
bytes.

The whole kvm x86 emulation thiing is quite complicated and has lots
of instruction size #defines and magic.

I'm not familiar enough with it to go "Ahh, it's obviously XYZ", but
I'm sure PeterZ and Borislav know exactly what's going on.

                 Linus