[{"id":3687491,"web_url":"http://patchwork.ozlabs.org/comment/3687491/","msgid":"<9794589b-0f4e-4f0a-9d67-ab7e9c1d749e@linaro.org>","list_archive_url":null,"date":"2026-05-07T04:58:48","subject":"Re: [PATCH 2/5] accel/tcg: move jit thread manipulation into\n do_tb_phys_invalidate","submitter":{"id":72104,"url":"http://patchwork.ozlabs.org/api/people/72104/","name":"Richard Henderson","email":"richard.henderson@linaro.org"},"content":"On 5/5/26 05:36, Alex Bennée wrote:\n> To invalidate a TB on MacOS we need to enable write access to the JIT\n> buffer. We were doing this for tb_phys_invalidate__locked but that is\n> not the only path into do_tb_phys_invalidate. Move the manipulation\n> into the shared function that does the work.\n> \n> This enables watchpoints to work in MacOS TCG guests.\n> \n> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3444\n> Signed-off-by: Alex Bennée \n> ---\n> accel/tcg/tb-maint.c | 5 +++--\n> 1 file changed, 3 insertions(+), 2 deletions(-)\n> \n> diff --git a/accel/tcg/tb-maint.c b/accel/tcg/tb-maint.c\n> index cd7c32361bb..9a648f97865 100644\n> --- a/accel/tcg/tb-maint.c\n> +++ b/accel/tcg/tb-maint.c\n> @@ -925,6 +925,7 @@ static void do_tb_phys_invalidate(TranslationBlock *tb, bool rm_from_page_list)\n> uint32_t orig_cflags = tb_cflags(tb);\n> \n> assert_memory_lock();\n> + qemu_thread_jit_write();\n> \n> /* make sure no further incoming jumps will be chained to this TB */\n> qemu_spin_lock(&tb->jmp_lock);\n> @@ -954,15 +955,15 @@ static void do_tb_phys_invalidate(TranslationBlock *tb, bool rm_from_page_list)\n> /* suppress any remaining jumps to this TB */\n> tb_jmp_unlink(tb);\n> \n> + qemu_thread_jit_execute();\n\nYou've missed the early return path from the middle of the function, which should be fatal.\n\nBut the place that needs this is tb_reset_jump, which is called from \ntb_remove_from_jmp_list and tb_jmp_unlink. Which is entirely covered by moving this down \ntoward the end of the function like so:\n\n---\n+ qemu_thread_jit_write();\n\n /* suppress this TB from the two jump lists */\n tb_remove_from_jmp_list(tb, 0);\n tb_remove_from_jmp_list(tb, 1);\n\n /* suppress any remaining jumps to this TB */\n tb_jmp_unlink(tb);\n\n+ qemu_thread_jit_execute();\n---\n\n\n> +\n> qatomic_set(&tb_ctx.tb_phys_invalidate_count,\n> tb_ctx.tb_phys_invalidate_count + 1);\n> }\n> \n> static void tb_phys_invalidate__locked(TranslationBlock *tb)\n> {\n> - qemu_thread_jit_write();\n> do_tb_phys_invalidate(tb, true);\n> - qemu_thread_jit_execute();\n> }\n\nMight as well remove tb_phys_invalidate__locked entirely, and propagate the direct call to \ndo_tb_phys_invalidate.\n\nThe __locked suffix does appear to be for the user assert_memory_locked(). As evidenced \nby tb_phys_invalidate, for system mode, we only sometimes take the page lock.\n\nGiven that this jit protection is via pthread_jit_write_protect_np, I assume the W^X \nprotection is a magic Apple per-thread bit. If so, we don't actually require cross-thread \nlocking at all, and all is well.\n\n\nr~","headers":{"Return-Path":"","X-Original-To":"incoming@patchwork.ozlabs.org","Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=linaro.org header.i=@linaro.org header.a=rsa-sha256\n header.s=google header.b=ZoDBF/HC;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; helo=lists1p.gnu.org;\n envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org;\n receiver=patchwork.ozlabs.org)"],"Received":["from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17])\n\t(using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits))\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4gB0RW0P4jz1yKd\n\tfor ; Thu, 07 May 2026 14:59:49 +1000 (AEST)","from localhost ([::1] helo=lists1p.gnu.org)\n\tby lists1p.gnu.org with esmtp (Exim 4.90_1)\n\t(envelope-from )\n\tid 1wKqp9-0006DK-Ez; 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