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Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON
From: |
Richard W.M. Jones |
Subject: |
Re: [PATCH] tcg/arm: Reduce vector alignment requirement for NEON |
Date: |
Mon, 13 Sep 2021 12:07:27 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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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