Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON

["Richard W.M. Jones" <rjones@redhat.com>]

On Sun, Sep 12, 2021 at 10:49:25AM -0700, Richard Henderson wrote:
> With arm32, the ABI gives us 8-byte alignment for the stack.
> While it's possible to realign the stack to provide 16-byte alignment,
> it's far easier to simply not encode 16-byte alignment in the
> VLD1 and VST1 instructions that we emit.
> 
> Remove the assertion in temp_allocate_frame, limit natural alignment
> to the provided stack alignment, and add a comment.
> 
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
> 
> I haven't seen the assertion with the various arm kernels that I happen
> to have laying about.  I have not taken the time to build the combo
> from the bug report:
> 
> [    0.000000] Linux version 5.14.0-60.fc36.armv7hl (mockbuild@buildvm-a32-12.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.37-9.fc36) #1 SMP Mon Aug 30 14:08:34 UTC 2021
> 
> I thought about parameterizing this patch further, but I can't think of
> another ISA that would be affected.  (i686 clumsily changed its abi 20
> years ago to avoid faulting on vector spills; other isas so far have
> allowed vectors to be unaligned.)

Is it possible this change could have caused a more serious
regression?  Now when I try to boot the Fedora kernel using TCG on
armv7hl I can't even get to the point where it detects virtio-scsi
devices.

Full log is here (go down to the bottom and work backwards):

  https://kojipkgs.fedoraproject.org//work/tasks/7337/75597337/build.log

This might have been caused by a coincidental change to the kernel.
The test environment I have makes it extremely difficult to test this
change in isolation.

However I do know that the same error does _not_ occur on x86-64
guest/host with this patch applied.

Rich.

> 
> r~
> ---
>  tcg/tcg.c                |  8 +++++++-
>  tcg/arm/tcg-target.c.inc | 13 +++++++++----
>  2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 4142d42d77..ca5bcc4635 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -3060,7 +3060,13 @@ static void temp_allocate_frame(TCGContext *s, TCGTemp *ts)
>          g_assert_not_reached();
>      }
>  
> -    assert(align <= TCG_TARGET_STACK_ALIGN);
> +    /*
> +     * Assume the stack is sufficiently aligned.
> +     * This affects e.g. ARM NEON, where we have 8 byte stack alignment
> +     * and do not require 16 byte vector alignment.  This seems slightly
> +     * easier than fully parameterizing the above switch statement.
> +     */
> +    align = MIN(TCG_TARGET_STACK_ALIGN, align);
>      off = ROUND_UP(s->current_frame_offset, align);
>  
>      /* If we've exhausted the stack frame, restart with a smaller TB. */
> diff --git a/tcg/arm/tcg-target.c.inc b/tcg/arm/tcg-target.c.inc
> index e5b4f86841..8515717435 100644
> --- a/tcg/arm/tcg-target.c.inc
> +++ b/tcg/arm/tcg-target.c.inc
> @@ -2477,8 +2477,13 @@ static void tcg_out_ld(TCGContext *s, TCGType type, TCGReg arg,
>          tcg_out_vldst(s, INSN_VLD1 | 0x7d0, arg, arg1, arg2);
>          return;
>      case TCG_TYPE_V128:
> -        /* regs 2; size 8; align 16 */
> -        tcg_out_vldst(s, INSN_VLD1 | 0xae0, arg, arg1, arg2);
> +        /*
> +         * We have only 8-byte alignment for the stack per the ABI.
> +         * Rather than dynamically re-align the stack, it's easier
> +         * to simply not request alignment beyond that.  So:
> +         * regs 2; size 8; align 8
> +         */
> +        tcg_out_vldst(s, INSN_VLD1 | 0xad0, arg, arg1, arg2);
>          return;
>      default:
>          g_assert_not_reached();
> @@ -2497,8 +2502,8 @@ static void tcg_out_st(TCGContext *s, TCGType type, TCGReg arg,
>          tcg_out_vldst(s, INSN_VST1 | 0x7d0, arg, arg1, arg2);
>          return;
>      case TCG_TYPE_V128:
> -        /* regs 2; size 8; align 16 */
> -        tcg_out_vldst(s, INSN_VST1 | 0xae0, arg, arg1, arg2);
> +        /* See tcg_out_ld re alignment: regs 2; size 8; align 8 */
> +        tcg_out_vldst(s, INSN_VST1 | 0xad0, arg, arg1, arg2);
>          return;
>      default:
>          g_assert_not_reached();
> -- 
> 2.25.1

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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