linux-kernel - Re: [PATCH 6/8] s390/boot: workaround llvm IAS bug

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Date:   Wed, 11 May 2022 12:50:45 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Jonas Paulsson <paulsson@...ux.vnet.ibm.com>,
        Ulrich Weigand <ulrich.weigand@...ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Andreas Krebbel <krebbel@...ux.ibm.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 6/8] s390/boot: workaround llvm IAS bug

On Wed, May 11, 2022 at 5:05 AM Heiko Carstens <hca@...ux.ibm.com> wrote:
>
> For at least the mvc and clc instructions llvm's integrated assembler can
> generate incorrect code. In particular this happens with decompressor boot
> code. The reason seems to be that relocations for the second displacement
> of each instruction are at incorrect locations (-/+: gas vs llvm IAS):
>
> mvc     __LC_IO_NEW_PSW(16),.Lnewpsw
>
> results in
>
>         4:      d2 0f 01 f0 00 00       mvc     496(16,%r0),0
> -                       8: R_390_12     .head.text+0x10
> +                       6: R_390_12     .head.text+0x10
>
> and
> clc     0(3,%r4),.L_hdr
> results in
>
>       258:      d5 02 40 00 00 00       clc     0(3,%r4),0
> -                       25c: R_390_12   .head.text+0x324
> +                       25a: R_390_12   .head.text+0x324
>
> Workaround this by writing the code in a different way.
>
> Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>

Please link to an LLVM bugreport for this.

> ---
>  arch/s390/boot/head.S | 34 +++++++++++++++++++++-------------
>  1 file changed, 21 insertions(+), 13 deletions(-)
>
> diff --git a/arch/s390/boot/head.S b/arch/s390/boot/head.S
> index 2ced90172680..8402e1cd133b 100644
> --- a/arch/s390/boot/head.S
> +++ b/arch/s390/boot/head.S
> @@ -42,7 +42,8 @@ ipl_start:
>  # subroutine to wait for end I/O
>  #
>  .Lirqwait:
> -       mvc     __LC_IO_NEW_PSW(16),.Lnewpsw    # set up IO interrupt psw
> +       larl    %r13,.Lnewpsw           # set up IO interrupt psw
> +       mvc     __LC_IO_NEW_PSW(16),0(%r13)
>         lpsw    .Lwaitpsw
>  .Lioint:
>         br      %r14
> @@ -155,9 +156,11 @@ ipl_start:
>         lr      %r2,%r3
>  .Lnotrunc:
>         l       %r4,.Linitrd
> -       clc     0(3,%r4),.L_hdr         # if it is HDRx
> +       larl    %r13,.L_hdr
> +       clc     0(3,%r4),0(%r13)        # if it is HDRx
>         bz      .Lagain1                # skip dataset header
> -       clc     0(3,%r4),.L_eof         # if it is EOFx
> +       larl    %r13,.L_eof
> +       clc     0(3,%r4),0(%r13)        # if it is EOFx
>         bz      .Lagain1                # skip dateset trailer
>
>         lr      %r5,%r2
> @@ -181,9 +184,11 @@ ipl_start:
>  .Lrdcont:
>         l       %r2,.Linitrd
>
> -       clc     0(3,%r2),.L_hdr         # skip HDRx and EOFx
> +       larl    %r13,.L_hdr             # skip HDRx and EOFx
> +       clc     0(3,%r2),0(%r13)
>         bz      .Lagain2
> -       clc     0(3,%r2),.L_eof
> +       larl    %r13,.L_eof
> +       clc     0(3,%r2),0(%r13)
>         bz      .Lagain2
>
>  #
> @@ -260,20 +265,23 @@ SYM_CODE_START_LOCAL(startup_normal)
>         .fill   16,4,0x0
>  0:     lmh     %r0,%r15,0(%r13)        # clear high-order half of gprs
>         sam64                           # switch to 64 bit addressing mode
> -       basr    %r13,0                  # get base
> -.LPG0:
> -       mvc     __LC_EXT_NEW_PSW(16),.Lext_new_psw-.LPG0(%r13)
> -       mvc     __LC_PGM_NEW_PSW(16),.Lpgm_new_psw-.LPG0(%r13)
> -       mvc     __LC_IO_NEW_PSW(16),.Lio_new_psw-.LPG0(%r13)
> +       larl    %r13,.Lext_new_psw
> +       mvc     __LC_EXT_NEW_PSW(16),0(%r13)
> +       larl    %r13,.Lpgm_new_psw
> +       mvc     __LC_PGM_NEW_PSW(16),0(%r13)
> +       larl    %r13,.Lio_new_psw
> +       mvc     __LC_IO_NEW_PSW(16),0(%r13)
>         xc      0x200(256),0x200        # partially clear lowcore
>         xc      0x300(256),0x300
>         xc      0xe00(256),0xe00
>         xc      0xf00(256),0xf00
> -       lctlg   %c0,%c15,.Lctl-.LPG0(%r13)      # load control registers
> +       larl    %r13,.Lctl
> +       lctlg   %c0,%c15,0(%r13)        # load control registers
>         stcke   __LC_BOOT_CLOCK
>         mvc     __LC_LAST_UPDATE_CLOCK(8),__LC_BOOT_CLOCK+1
> -       spt     6f-.LPG0(%r13)
> -       mvc     __LC_LAST_UPDATE_TIMER(8),6f-.LPG0(%r13)
> +       larl    %r13,6f
> +       spt     0(%r13)
> +       mvc     __LC_LAST_UPDATE_TIMER(8),0(%r13)
>         larl    %r15,_stack_end-STACK_FRAME_OVERHEAD
>         brasl   %r14,sclp_early_setup_buffer
>         brasl   %r14,verify_facilities
> --
> 2.32.0
>


-- 
Thanks,
~Nick Desaulniers