linux-kernel - Re: [PATCH 6.10 000/809] 6.10.3-rc3 review

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Message-ID: <250f3ae6-3a81-40c7-a747-4713e8888510@gmx.de>
Date: Tue, 3 Sep 2024 09:54:19 +0200
From: Helge Deller <deller@....de>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Richard Henderson <richard.henderson@...aro.org>,
 Guenter Roeck <linux@...ck-us.net>
Cc: Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
 Linux-MM <linux-mm@...ck.org>, linux-parisc@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6.10 000/809] 6.10.3-rc3 review

On 8/8/24 20:19, Linus Torvalds wrote:
> On Thu, 8 Aug 2024 at 10:48, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> Here is the disassembly from my latest crashing debug kernel which
>> shifts it up a couple of pages. Add 0x10 or sub 0x20 to make it work.
>
> Looks like I was off by an instruction, it's the 28th divide-step (not
> 29) that does the page crosser:
>
>>      4121dffc:   0b 21 04 41     ds r1,r25,r1
>>      4121e000:   0b bd 07 1d     add,c ret1,ret1,ret1
>
> but my parisc knowledge is not good enough to even guess at what could go wrong.
>
> And I have no actual reason to believe this has *anything* to do with
> an itlb miss, except for that whole "exact placement seems to matter,
> and it crosses a page boundary" detail.

Well, you were on the right track :-)

Guenters kernel from
http://server.roeck-us.net/qemu/parisc64-6.10.3/
boots nicely on my physical C3700 machine, but crashes with Qemu.

So, it's not some bug in the kernel ITLB miss handler or other
Linux kernel code.

Instead it's a Qemu bug, which gets triggered by the page
boundary crossing of:
    41218ffc:   0b 21 04 41     ds r1,r25,r1
    41219000:   0b bd 07 1d     add,c ret1,ret1,ret1

During the ITLB miss, the carry bits and the PSW-V-bit
(from the divide step) are saved in the IPSW register and restored
upon irq return.

During packaging the bits there is a qemu coding bug, where we missed
to handle the PSW-V-bit as 32-bit value even on a 64-bit CPU.
The (copy&pasted) patch below fixes the crash for me.

Helge

diff --git a/target/hppa/helper.c b/target/hppa/helper.c
index b79ddd8184..d4b1a3cd5a 100644
--- a/target/hppa/helper.c
+++ b/target/hppa/helper.c
@@ -53,7 +53,7 @@ target_ulong cpu_hppa_get_psw(CPUHPPAState *env)
      }

      psw |= env->psw_n * PSW_N;
-    psw |= (env->psw_v < 0) * PSW_V;
+    psw |= ((env->psw_v >> 31) & 1) * PSW_V;
      psw |= env->psw | env->psw_xb;

      return psw;