Date: Fri, 16 Jul 2021 17:07:42 +0200 From: Michael Tuexen <tuexen@freebsd.org> To: Andrew Turner <andrew@fubar.geek.nz> Cc: Mark Millard <marklmi@yahoo.com>, freebsd-arm@freebsd.org Subject: Re: register x18 Message-ID: <4361A215-BB47-4166-BC3F-386E7834B788@freebsd.org> In-Reply-To: <ACD1D84A-5923-4106-AAE4-35FB7A182B0F@fubar.geek.nz> References: <86EC9C12-F90C-4D0C-BFA3-41986C9F07B5@freebsd.org> <BFF3BCE7-3387-4A7C-A71C-890223CDDF18@yahoo.com> <32C24DDC-C8A1-43CD-9220-8009B229E452@freebsd.org> <ACD1D84A-5923-4106-AAE4-35FB7A182B0F@fubar.geek.nz>
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> On 16. Jul 2021, at 14:51, Andrew Turner <andrew@fubar.geek.nz> wrote: >=20 >=20 >> On 16 Jul 2021, at 13:08, tuexen@freebsd.org wrote: >>=20 >>> On 16. Jul 2021, at 04:06, Mark Millard <marklmi@yahoo.com> wrote: >>>=20 >>>=20 >>>=20 >>> On 2021-Jul-15, at 17:40, Michael Tuexen <tuexen at freebsd.org> = wrote: >>>=20 >>>> Dear all, >>>>=20 >>>> register x18 seems to be special. What is it used for in FreeBSD? >>>>=20 >>>> Best regards >>>> Michael >>>=20 >>> = https://developer.arm.com/documentation/den0024/a/The-ABI-for-ARM-64-bit-A= rchitecture/Register-use-in-the-AArch64-Procedure-Call-Standard/Parameters= -in-general-purpose-registers >>>=20 >>> reports: >>>=20 >>> QUOTE >>> =E2=80=A2 X18 is the platform register and is reserved for the = use of platform ABIs. This is an adional temporary register on platforms = that don't assign a special meaning to it. >>> END QUOTE >>>=20 >>> So, special, yes. But I do not know what the "platform ABI" usage >>> for it might be on FreeBSD. So, for the most part, this does not >>> well-answer your question. Sorry. >> Yepp, I found the above text. However, x18 seems to be used when = accessing >> global variables. I am looking at a panic, where the system panics on = accessing >> global variable, which can be controlled by sysctl. >> It seems that x18 does not have the expected value, but it is also = not set in >> the function... >=20 > X18 is used to store the pointer to the pcpu data It should only ever = be set when we enter the kernel from userland by the exception handler. Hi Andrew, thanks for the response. Hmm. I was hoping that the answers helps me to = understand a panic that I'm observing when stress testing the TCP RACK stack. I'm = transferring 10GB via scp and at some point of time (not right at the beginning), the = machine panics. The machine is an eMAG system. Here is what I know: Initially it panics multiple times (always at the same place) in https://cgit.freebsd.org/src/tree/sys/netinet/tcp_stacks/rack.c#n16540 when it is trying to read V_tcp_map_entries_limit. I discussed this with rrs@ and since we had no clue, I tried to just = compile out the if condition. Then is paniced in https://cgit.freebsd.org/src/tree/sys/netinet/tcp_stacks/rack.c#n16928 at https://cgit.freebsd.org/src/tree/sys/netinet/tcp_stacks/rack.c#n15664 which is basically the next place where a V_ variable is accessed. Please note that for debugging I'm using a kernel without VIMAGE = support, since we initially thought that it might be related a VNET bug. So I decided to look at the disassembly of rack_sndbuf_autoscale (I = added some comments): 0xffff000001388a6c <+0>: stp x29, x30, [sp, #-32]! 0xffff000001388a70 <+4>: str x19, [sp, #16] 0xffff000001388a74 <+8>: mov x29, sp 0xffff000001388a78 <+12>: ldr x9, [x0, #24] = // x9 =3D rack->tp; 0xffff000001388a7c <+16>: ldr w8, [x0, #188] = // w8 =3D rack->r_ctl.cwnd_to_use 0xffff000001388a80 <+20>: adrp x12, 0xffff0000013ac000 0xffff000001388a84 <+24>: ldr w10, [x9, #52] = // w10 =3D tp->snd_wnd; 0xffff000001388a88 <+28>: ldr x11, [x18] 0xffff000001388a8c <+32>: ldr x11, [x11, #1256] 0xffff000001388a90 <+36>: cmp w8, w10 0xffff000001388a94 <+40>: csel w10, w8, w10, cc // cc =3D lo, = ul, last // min(rack->r_ctl.cwnd_to_use, tp->snd_wnd); =3D> 0xffff000001388a98 <+44>: ldr x11, [x11, #40] 0xffff000001388a9c <+48>: ldr x12, [x12, #2752] 0xffff000001388aa0 <+52>: ldr w11, [x11, x12] = // w11 =3D V_tcp_do_autosndbuf ??? 0xffff000001388aa4 <+56>: cbz w11, 0xffff000001388be0 = <rack_sndbuf_autoscale+372> 0xffff000001388aa8 <+60>: ldr x8, [x0, #32] = // x8 =3D rack->rc_inp 0xffff000001388aac <+64>: ldr x19, [x8, #120] = // x19 =3D so =3D x8->inp_socket 0xffff000001388ab0 <+68>: ldrb w8, [x19, #817] = // w8 =3D (x19->so_snd.sb_flags << 8) & 0ff 0xffff000001388ab4 <+72>: tbz w8, #3, 0xffff000001388be0 = <rack_sndbuf_autoscale+372> so->so_snd.sb_flags & SB_AUTOSIZE =3D=3D 0 0xffff000001388ab8 <+76>: ldr w11, [x9, #52] = // w11 =3D tp->snd_wnd 0xffff000001388abc <+80>: ldr w8, [x19, #740] = // w8 =3D so->so_snd.sb_hiwat 0xffff000001388ac0 <+84>: lsr w11, w11, #2 0xffff000001388ac4 <+88>: add w11, w11, w11, lsl #2 0xffff000001388ac8 <+92>: cmp w11, w8 0xffff000001388acc <+96>: b.cc 0xffff000001388be0 = <rack_sndbuf_autoscale+372> // b.lo, b.ul, b.last 0xffff000001388ad0 <+100>: ldr w11, [x19, #736] 0xffff000001388ad4 <+104>: lsr w8, w8, #3 0xffff000001388ad8 <+108>: lsl w12, w8, #3 0xffff000001388adc <+112>: sub w8, w12, w8 0xffff000001388ae0 <+116>: cmp w11, w8 0xffff000001388ae4 <+120>: b.cc 0xffff000001388be0 = <rack_sndbuf_autoscale+372> // b.lo, b.ul, b.last 0xffff000001388ae8 <+124>: ldr x8, [x18] 0xffff000001388aec <+128>: ldr x8, [x8, #1256] 0xffff000001388af0 <+132>: ldr x12, [x8, #40] 0xffff000001388af4 <+136>: adrp x8, 0xffff0000013ac000 0xffff000001388af8 <+140>: ldr x8, [x8, #2760] 0xffff000001388afc <+144>: ldr w12, [x12, x8] 0xffff000001388b00 <+148>: cmp w11, w12 So it seems that the code accessing V_tcp_do_autosndbuf is: 0xffff000001388a80 <+20>: adrp x12, 0xffff0000013ac000 ... 0xffff000001388a88 <+28>: ldr x11, [x18] 0xffff000001388a8c <+32>: ldr x11, [x11, #1256] ... =3D> 0xffff000001388a98 <+44>: ldr x11, [x11, #40] 0xffff000001388a9c <+48>: ldr x12, [x12, #2752] 0xffff000001388aa0 <+52>: ldr w11, [x11, x12] = // w11 =3D V_tcp_do_autosndbuf ??? and for V_tcp_autosndbuf_max it is: 0xffff000001388ae8 <+124>: ldr x8, [x18] 0xffff000001388aec <+128>: ldr x8, [x8, #1256] 0xffff000001388af0 <+132>: ldr x12, [x8, #40] 0xffff000001388af4 <+136>: adrp x8, 0xffff0000013ac000 0xffff000001388af8 <+140>: ldr x8, [x8, #2760] 0xffff000001388afc <+144>: ldr w12, [x12, x8] The #2752 versus #2760 could be the offset of the variable. Does the above code makes sense to you? The code relevant for the crash = seems to be: 0xffff000001388a88 <+28>: ldr x11, [x18] 0xffff000001388a8c <+32>: ldr x11, [x11, #1256] 0xffff000001388a98 <+44>: ldr x11, [x11, #40] Since it is crashing at 0xffff000001388a98 <+44>, my assumption was that = x18 is wrong... But does this use fit to your description? I'm trying to debug this on arm64, since I can reproduce it on arm64. = But there is also a bug report that this happens on amd64: = https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D257195 Any idea what can be wrong? Any hint how to progress? Thank you very much for your help! Best regards Michael >=20 > Andrew
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