[{"id":3681888,"web_url":"http://patchwork.ozlabs.org/comment/3681888/","msgid":"","list_archive_url":null,"date":"2026-04-24T08:57:11","subject":"Re: [PATCH] build: prefer sigaltstack over ucontext on Linux","submitter":{"id":2694,"url":"http://patchwork.ozlabs.org/api/people/2694/","name":"Daniel P. Berrangé","email":"berrange@redhat.com"},"content":"On Fri, Apr 24, 2026 at 11:19:39AM +0300, Elisey Konstantinov wrote:\n> From 517099e65de1e2ad3d0f33d3c4a7a71f8728279c Mon Sep 17 00:00:00 2001\n> From: Elisey \n> Date: Mon, 23 Mar 2026 16:02:45 +0100\n> Subject: [PATCH] build: prefer sigaltstack over ucontext on Linux\n> \n> sigaltstack coroutine backend is more stable on some Linux\n> configurations, while ucontext causes boot failures for certain\n> PPC Mac OS X guests (issue #3276).\n> Select sigaltstack first when available, keeping ucontext as fallback\n> until the underlying issue is resolved.\n\nIMHO there needs to be a greater investigation in to why ucontext\nwould be demonstrating the failure. ucontext has been the default\non Linux for QEMU for decades without significant issues, so my\nconcern is that switching to sigaltstack is not fixing the root\ncause of the bug you see, just masking it by luck.\n\nThe choice of coroutine backend can also be performance sensitive\nand so changing it is something that would need to also bring\nalong performance benchmark data to illustrate the likely impact\nof the change.\n\n> \n> Signed-off-by: Elisey Konstantinov \n> ---\n> meson.build | 4 +++-\n> 1 file changed, 3 insertions(+), 1 deletion(-)\n> \n> diff --git a/meson.build b/meson.build\n> index ab3e97eb9f..742655e007 100644\n> --- a/meson.build\n> +++ b/meson.build\n> @@ -505,10 +505,12 @@ if host_os == 'windows'\n> elif host_os == 'emscripten'\n> supported_backends += ['wasm']\n> else\n> + # FIXME: This is a temporary workaround!\n> + # SigAltStack is in higher priority to fix #3276\n> + supported_backends += ['sigaltstack']\n> if host_os != 'darwin' and cc.links(ucontext_probe)\n> supported_backends += ['ucontext']\n> endif\n> - supported_backends += ['sigaltstack']\n> endif\n> \n> if coroutine_backend == 'auto'\n> -- \n> 2.50.1 (Apple Git-155)\n>","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 (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=KIbr8M01;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=google header.b=pR41Gf0q;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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charset=utf-8","Content-Disposition":"inline","In-Reply-To":"<616E96C0-F0A5-4A9E-A63B-3BC1B5F18094@gmail.com>","User-Agent":"Mutt/2.3.1 (2026-03-20)","Received-SPF":"pass client-ip=170.10.129.124;\n envelope-from=berrange@redhat.com;\n helo=us-smtp-delivery-124.mimecast.com","X-Spam_score_int":"-20","X-Spam_score":"-2.1","X-Spam_bar":"--","X-Spam_report":"(-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001,\n DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1,\n RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001,\n SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Reply-To":"Daniel =?utf-8?b?UC4gQmVycmFuZ8Op?= ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"}},{"id":3681908,"web_url":"http://patchwork.ozlabs.org/comment/3681908/","msgid":"<06C2CFB8-BA33-46D6-9F84-B1F81A2A319E@gmail.com>","list_archive_url":null,"date":"2026-04-24T09:19:19","subject":"Re: [PATCH] build: prefer sigaltstack over ucontext on Linux","submitter":{"id":93247,"url":"http://patchwork.ozlabs.org/api/people/93247/","name":"Elisey Konstantinov","email":"elisey.konstantinov@gmail.com"},"content":"I completely agree, but in my opinion, investigating UContext will take a considerable amount of time, which is why I took the liberty of proposing this patch; in the commit message, I actually mention that this is merely a “temporary workaround”\n\nSo, I suggest:\n- Merging the patch into the mainstream for the convenience of users (as, from what I’ve observed, SigAltStack is coping quite well with the task, at least for now...)\n- Whilst leaving #3276 open for the time being and continuing the UContext diagnostics there\n- And accordingly, as soon as UContext is fully functional, we can switch QEMU back from SigAltStack\n\nAlso the another arrangement is to use UContext ONLY if PPC is in the build targets, but I am unable to do this, as my knowledge of Meson is insufficient for such a task; nevertheless, I would be grateful for any assistance :)\n\nP. S. Anticipating such a reaction, I DELIBERATELY did not include “Resolves” in the patch notes, so I am fully aware of all the proposed precautions","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=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=IS6h0o/Y;\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 4g26qw5p3vz1yD5\n\tfor ; Fri, 24 Apr 2026 19:20:11 +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 1wGCh9-0005bE-65; 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Berrangé","email":"berrange@redhat.com"},"content":"On Fri, Apr 24, 2026 at 12:19:19PM +0300, Elisey Konstantinov wrote:\n> I completely agree, but in my opinion, investigating UContext will take a considerable amount of time, which is why I took the liberty of proposing this patch; in the commit message, I actually mention that this is merely a “temporary workaround”\n> \n> So, I suggest:\n> - Merging the patch into the mainstream for the convenience of users (as, from what I’ve observed, SigAltStack is coping quite well with the task, at least for now...)\n> - Whilst leaving #3276 open for the time being and continuing the UContext diagnostics there\n> - And accordingly, as soon as UContext is fully functional, we can switch QEMU back from SigAltStack\n> \n> Also the another arrangement is to use UContext ONLY if PPC is in the build targets, but I am unable to do this, as my knowledge of Meson is insufficient for such a task; nevertheless, I would be grateful for any assistance :)\n> \n> P. S. Anticipating such a reaction, I DELIBERATELY did not include “Resolves” in the patch notes, so I am fully aware of all the proposed precautions\n\nPPC MacOS guest is such a niche use case, that I don't see a compelling\nreason to make such a change in git until the ticket has been fully\ninvestigated and the problem root caused.","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 (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=bJSFQAcc;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=google header.b=PW53geS/;\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 4g276C2Gm7z1yDD\n\tfor ; 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S.\n\tFor example, SigAltStack is the only one way to use QEMU on macOS, and it works.. pretty good\n\tTo be clear, only Linux and maybe BSD (which I can’t test, see details in the issue) works on UContext by the meantime...","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=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=Wc5i/+1X;\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 4g29mQ0vCBz1yDD\n\tfor ; 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helo=mail-wr1-x431.google.com","X-Spam_score_int":"-10","X-Spam_score":"-1.1","X-Spam_bar":"-","X-Spam_report":"(-1.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FORGED_GMAIL_RCVD=1,\n FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001,\n SPF_PASS=-0.001 autolearn=no autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"}},{"id":3681996,"web_url":"http://patchwork.ozlabs.org/comment/3681996/","msgid":"","list_archive_url":null,"date":"2026-04-24T11:51:14","subject":"Re: [PATCH] build: prefer sigaltstack over ucontext on Linux","submitter":{"id":5111,"url":"http://patchwork.ozlabs.org/api/people/5111/","name":"Peter Maydell","email":"peter.maydell@linaro.org"},"content":"On Fri, 24 Apr 2026 at 12:32, Elisey Konstantinov\n wrote:\n>\n> I understand it’s a niche use case, however, “niche” doesn't mean “broken”\n> Currently, QEMU advertises support for PPC Mac OS X guests, but it doesn’t work on Linux\n> for certain versions\n> My patch fixes this breakage with a 3-line change, proven to work\n>\n> If the root cause in UContext takes months to fix, will you leave the feature broken for all Linux users that long?\n> Can we apply this workaround to unblock users now, and continue the investigation in the issue tracker?\n\nQEMU covers many more use cases than PPC macos emulation.\nAs Daniel says, this is very niche. It's almost certainly the\ncase that your proposed change is merely hiding an actual bug.\nThe coroutine backend is a fundamental part of how low level\nparts of QEMU like the block backend work, which are heavily\nrelied on where QEMU is used as a virtual machine setup with\nKVM and other accelerators. Changing the default here affects\na great many of our users. We should not do that only because\nit happens to mask a bug somewhere in PPC emulation for\na particular guest OS.\n\nIf this was a workaround that affected only PPC and was at least\nobviously a change to a PPC-specific part of QEMU like a particular\ndevice model, we might take it. (Though even there we might say \"no,\nfind the actual cause\"; workarounds once accepted have a tendency\nto stick around way longer than they ought to as people forget\nwhether and when they can be removed later). For a workaround which\naffects every QEMU user including all the performance-sensitive\nVM use cases, the answer is a clear \"no\".\n\n> If not, what is the official status of PPC Mac OS X support in QEMU?\n\nThe \"New World\" and \"Old World\" machines are listed in our\nMAINTAINERS file as \"Odd Fixes\", which means:\n\n\"It has a maintainer but they don't have time to do\n much other than throw the odd patch in.\"\n\nIn other words, there is no active development work ongoing,\nand nobody with time and interest to investigate complex bugs.\nIf it happens to work for people, that's great. If it doesn't\nwork and somebody proposes a patch then we can review the patch\n(as we have done for this one). If it doesn't work and nobody\nproposes a satisfactory patch then it will likely remain broken.\n\nthanks\n-- PMM","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=AeM2f7Ri;\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 4g2BCN1gSpz1yD5\n\tfor ; Fri, 24 Apr 2026 21:52:14 +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 1wGF4M-0005Cm-L3; 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Fri, 24 Apr 2026\n 04:51:25 -0700 (PDT)","MIME-Version":"1.0","References":"<616E96C0-F0A5-4A9E-A63B-3BC1B5F18094@gmail.com>\n \n <06C2CFB8-BA33-46D6-9F84-B1F81A2A319E@gmail.com>\n \n <4FFF1823-815E-4956-9C80-9C62A98033B0@gmail.com>","In-Reply-To":"<4FFF1823-815E-4956-9C80-9C62A98033B0@gmail.com>","From":"Peter Maydell ","Date":"Fri, 24 Apr 2026 12:51:14 +0100","X-Gm-Features":"AQROBzBMy2AjmO9dx-K505fJ9xL3QVkDzIqOIqaZOywoLlD6pbefOYOI8InzlQk","Message-ID":"\n ","Subject":"Re: [PATCH] build: prefer sigaltstack over ucontext on Linux","To":"Elisey Konstantinov ","Cc":"=?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= ,\n qemu-devel@nongnu.org, pbonzini@redhat.com, marcandre.lureau@redhat.com,\n philmd@linaro.org","Content-Type":"text/plain; charset=\"UTF-8\"","Content-Transfer-Encoding":"quoted-printable","Received-SPF":"pass client-ip=2607:f8b0:4864:20::112d;\n envelope-from=peter.maydell@linaro.org; helo=mail-yw1-x112d.google.com","X-Spam_score_int":"-20","X-Spam_score":"-2.1","X-Spam_bar":"--","X-Spam_report":"(-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1,\n RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001,\n SPF_PASS=-0.001 autolearn=ham autolearn_force=no","X-Spam_action":"no action","X-BeenThere":"qemu-devel@nongnu.org","X-Mailman-Version":"2.1.29","Precedence":"list","List-Id":"qemu development ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org","Sender":"qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org"}},{"id":3682005,"web_url":"http://patchwork.ozlabs.org/comment/3682005/","msgid":"<175446b9-a3c3-4a20-bb6b-244867ea0022@redhat.com>","list_archive_url":null,"date":"2026-04-24T12:03:23","subject":"Re: [PATCH] build: prefer sigaltstack over ucontext on Linux","submitter":{"id":2701,"url":"http://patchwork.ozlabs.org/api/people/2701/","name":"Paolo Bonzini","email":"pbonzini@redhat.com"},"content":"On 4/24/26 09:19, Elisey Konstantinov wrote:\n> From 517099e65de1e2ad3d0f33d3c4a7a71f8728279c Mon Sep 17 00:00:00 2001\n> From: Elisey \n> Date: Mon, 23 Mar 2026 16:02:45 +0100\n> Subject: [PATCH] build: prefer sigaltstack over ucontext on Linux\n> \n> sigaltstack coroutine backend is more stable on some Linux\n> configurations, while ucontext causes boot failures for certain\n> PPC Mac OS X guests (issue #3276).\n> \n> Select sigaltstack first when available, keeping ucontext as fallback\n> until the underlying issue is resolved.\n> \n> Signed-off-by: Elisey Konstantinov \n\nI have never really liked using ucontext because we are only using it \nfor setup and then switching to sigsetjmp/siglongjmp. sigaltstack is \nnot much better however. I agree with others that for example the mutex \ncould be masking the bug.\n\nMany years ago I wrote an assembly-language coroutine backend:\n\nhttps://patchew.org/QEMU/20190504120528.6389-1-pbonzini@redhat.com/\n\nPerhaps you can try to resurrect it, even just for x86 to see if it \nfixes your issue? This is the relevant patch:\n\nhttps://patchew.org/QEMU/20190504120528.6389-4-pbonzini@redhat.com/mbox\n\nand you only need util/coroutine-asm.c + simple changes to meson.build \nsimilar to the ones you have already made.\n\nNote that you'll also have to add something like\n\ndiff --git a/meson.build b/meson.build\nindex 2bc2bd4d7bf..3a13da8a08d 100644\n--- a/meson.build\n+++ b/meson.build\n@@ -391,6 +391,7 @@ qemu_isa_flags = []\n\n # Pick x86-64 baseline version\n if host_arch == 'x86_64'\n+ qemu_isa_flags += ['-mno-red-zone']\n if get_option('x86_version') == '0'\n error('x86_64-v1 required for x86-64 hosts')\n endif\n\nto meson.build (not optimal but simple).\n\nPaolo","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 (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=PaiQjsw1;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; 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