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

[Farhan Ali]

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

Yes.

> 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)
>          UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER
>          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)
>   .Lsysc_do_svc:
> +       # 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 
> critical
> 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 
> > > different.
> > >
> > >
> > > (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