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Re: [qemu-s390x] [Qemu-devel] [BUG] I/O thread segfault for QEMU on s390

From: Farhan Ali
Subject: Re: [qemu-s390x] [Qemu-devel] [BUG] I/O thread segfault for QEMU on s390x
Date: Mon, 5 Mar 2018 14:43:44 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 03/05/2018 02:08 PM, Christian Borntraeger wrote:
Do you happen to run with a recent host kernel that has

commit 7041d28115e91f2144f811ffe8a195c696b1e1d0
     s390: scrub registers on kernel entry and KVM exit


Can you run with this on top
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index 13a133a6015c..d6dc0e5e8f74 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -426,13 +426,13 @@ ENTRY(system_call)
         BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP
         stmg    %r0,%r7,__PT_R0(%r11)
-       # clear user controlled register to prevent speculative use
-       xgr     %r0,%r0
         mvc     __PT_R8(64,%r11),__LC_SAVE_AREA_SYNC
         mvc     __PT_PSW(16,%r11),__LC_SVC_OLD_PSW
         mvc     __PT_INT_CODE(4,%r11),__LC_SVC_ILC
         stg     %r14,__PT_FLAGS(%r11)
+       # clear user controlled register to prevent speculative use
+       xgr     %r0,%r0
         # load address of system call table
         lg      %r10,__THREAD_sysc_table(%r13,%r12)
         llgh    %r8,__PT_INT_CODE+2(%r11)

To me it looks like that the critical section cleanup (interrupt during system 
call entry) might
save the registers again into ptregs but we have already zeroed out r0.
This patch moves the clearing of r0 after sysc_do_svc, which should fix the 
section cleanup.

Okay I will run with this.

Adding Martin and Heiko. Will spin a patch.

On 03/05/2018 07:54 PM, Christian Borntraeger wrote:

On 03/05/2018 07:45 PM, Farhan Ali wrote:

On 03/05/2018 06:03 AM, Stefan Hajnoczi wrote:
Please include the following gdb output:

    (gdb) disas swapcontext
    (gdb) i r

That way it's possible to see which instruction faulted and which
registers were being accessed.

here is the disas out for swapcontext, this is on a coredump with debugging 
symbols enabled for qemu. So the addresses from the previous dump is a little 

(gdb) disas swapcontext
Dump of assembler code for function swapcontext:
    0x000003ff90751fb8 <+0>:    lgr    %r1,%r2
    0x000003ff90751fbc <+4>:    lgr    %r0,%r3
    0x000003ff90751fc0 <+8>:    stfpc    248(%r1)
    0x000003ff90751fc4 <+12>:    std    %f0,256(%r1)
    0x000003ff90751fc8 <+16>:    std    %f1,264(%r1)
    0x000003ff90751fcc <+20>:    std    %f2,272(%r1)
    0x000003ff90751fd0 <+24>:    std    %f3,280(%r1)
    0x000003ff90751fd4 <+28>:    std    %f4,288(%r1)
    0x000003ff90751fd8 <+32>:    std    %f5,296(%r1)
    0x000003ff90751fdc <+36>:    std    %f6,304(%r1)
    0x000003ff90751fe0 <+40>:    std    %f7,312(%r1)
    0x000003ff90751fe4 <+44>:    std    %f8,320(%r1)
    0x000003ff90751fe8 <+48>:    std    %f9,328(%r1)
    0x000003ff90751fec <+52>:    std    %f10,336(%r1)
    0x000003ff90751ff0 <+56>:    std    %f11,344(%r1)
    0x000003ff90751ff4 <+60>:    std    %f12,352(%r1)
    0x000003ff90751ff8 <+64>:    std    %f13,360(%r1)
    0x000003ff90751ffc <+68>:    std    %f14,368(%r1)
    0x000003ff90752000 <+72>:    std    %f15,376(%r1)
    0x000003ff90752004 <+76>:    slgr    %r2,%r2
    0x000003ff90752008 <+80>:    stam    %a0,%a15,184(%r1)
    0x000003ff9075200c <+84>:    stmg    %r0,%r15,56(%r1)
    0x000003ff90752012 <+90>:    la    %r2,2
    0x000003ff90752016 <+94>:    lgr    %r5,%r0
    0x000003ff9075201a <+98>:    la    %r3,384(%r5)
    0x000003ff9075201e <+102>:    la    %r4,384(%r1)
    0x000003ff90752022 <+106>:    lghi    %r5,8
    0x000003ff90752026 <+110>:    svc    175

sys_rt_sigprocmask. r0 should not be changed by the system call.

    0x000003ff90752028 <+112>:    lgr    %r5,%r0
=> 0x000003ff9075202c <+116>:    lfpc    248(%r5)

so r5 is zero and it was loaded from r0. r0 was loaded from r3 (which is the 
2nd parameter to this
function). Now this is odd.

    0x000003ff90752030 <+120>:    ld    %f0,256(%r5)
    0x000003ff90752034 <+124>:    ld    %f1,264(%r5)
    0x000003ff90752038 <+128>:    ld    %f2,272(%r5)
    0x000003ff9075203c <+132>:    ld    %f3,280(%r5)
    0x000003ff90752040 <+136>:    ld    %f4,288(%r5)
    0x000003ff90752044 <+140>:    ld    %f5,296(%r5)
    0x000003ff90752048 <+144>:    ld    %f6,304(%r5)
    0x000003ff9075204c <+148>:    ld    %f7,312(%r5)
    0x000003ff90752050 <+152>:    ld    %f8,320(%r5)
    0x000003ff90752054 <+156>:    ld    %f9,328(%r5)
    0x000003ff90752058 <+160>:    ld    %f10,336(%r5)
    0x000003ff9075205c <+164>:    ld    %f11,344(%r5)
    0x000003ff90752060 <+168>:    ld    %f12,352(%r5)
    0x000003ff90752064 <+172>:    ld    %f13,360(%r5)
    0x000003ff90752068 <+176>:    ld    %f14,368(%r5)
    0x000003ff9075206c <+180>:    ld    %f15,376(%r5)
    0x000003ff90752070 <+184>:    lam    %a2,%a15,192(%r5)
    0x000003ff90752074 <+188>:    lmg    %r0,%r15,56(%r5)
    0x000003ff9075207a <+194>:    br    %r14
End of assembler dump.

(gdb) i r
r0             0x0    0
r1             0x3ff8fe7de40    4396165881408
r2             0x0    0
r3             0x3ff8fe7e1c0    4396165882304
r4             0x3ff8fe7dfc0    4396165881792
r5             0x0    0
r6             0xffffffff88004880    18446744071696304256
r7             0x3ff880009e0    4396033247712
r8             0x27ff89000    10736930816
r9             0x3ff88001460    4396033250400
r10            0x1000    4096
r11            0x1261be0    19274720
r12            0x3ff88001e00    4396033252864
r13            0x14d0bc0    21826496
r14            0x1312ac8    19999432
r15            0x3ff8fe7dc80    4396165880960
pc             0x3ff9075202c    0x3ff9075202c <swapcontext+116>
cc             0x2    2

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