From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EE63EC433EF for ; Wed, 4 May 2022 14:44:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1351437AbiEDOrr (ORCPT ); Wed, 4 May 2022 10:47:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58716 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344416AbiEDOrq (ORCPT ); Wed, 4 May 2022 10:47:46 -0400 Received: from mail-oa1-x34.google.com (mail-oa1-x34.google.com [IPv6:2001:4860:4864:20::34]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 612BEDF88 for ; Wed, 4 May 2022 07:44:09 -0700 (PDT) Received: by mail-oa1-x34.google.com with SMTP id 586e51a60fabf-edeb6c3642so1386969fac.3 for ; Wed, 04 May 2022 07:44:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digitalocean.com; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=hzXxKtcQps0ZAgRhahbmep1urQpR9Cgb4jllU0BGEjo=; b=AaRIzJr666XG5+EfWEqXud0jEDdbtOXM3arp9tH/ET8Mu61UpwavXLlLZgQxIODP2d 4DshXJXHjPS28WuTdLfDzanffOSPGmzwsLqHAeWqGxfPYGzrxvrxuBcvBmiJmcvkQTSF R3+3lAqSD3p+J4kIUAOaWa6OJlMzCRsSFoqpI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=hzXxKtcQps0ZAgRhahbmep1urQpR9Cgb4jllU0BGEjo=; b=LR8dW5cC+CU0ZAy0QtPmkB0tvCG/11pzS4qboAcYML748sTHFWhkLuiNf8IySU+myu sqf+37dj3IYn9/zzxYwZovcqDP56zPJ+2IrKtQUs5eJDnS2Q6auMtzr9TSNlayYU+IaB EEvIY2fnZ3/krzN4DDmOddUD2dEqsR44pctClog97ubaTm5B7yFOIaxbN/+9304Se5sX cPV26kyOb/MUEGyQEoA1dVS697JqmIaLzZRj0WvahkitMfMVachWEv7L1zKttOZ4FLct wwqXfedAg4tRs9mJdruRpmb7My0YWSAk1PBMv2AMzF/9RSX6U70/x5KTMnxNXATnN+BW r1BQ== X-Gm-Message-State: AOAM533d+gDcTPdUH+73Ok7aIAgr9UzPBmBkHUS578CsVPjWvdQTEiVN EdacH1T7TemdRow79U9XikZhhA== X-Google-Smtp-Source: ABdhPJx3/UeuBZsluNWODA4kiKaqDvbK5qLaNKbUKByLbstg23zONbpgcCwRsLd3dXF4WjHRvUdLqw== X-Received: by 2002:a05:6870:eaa5:b0:da:b3f:2b45 with SMTP id s37-20020a056870eaa500b000da0b3f2b45mr3979241oap.228.1651675448541; Wed, 04 May 2022 07:44:08 -0700 (PDT) Received: from localhost ([2605:a601:ac0f:820:373b:a889:93d6:e756]) by smtp.gmail.com with ESMTPSA id h3-20020a056870d24300b000e686d1386fsm8064357oac.9.2022.05.04.07.44.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 May 2022 07:44:08 -0700 (PDT) Date: Wed, 4 May 2022 09:44:07 -0500 From: Seth Forshee To: Petr Mladek Cc: Thomas Gleixner , Peter Zijlstra , Andy Lutomirski , Josh Poimboeuf , Jiri Kosina , Miroslav Benes , Paolo Bonzini , "Eric W. Biederman" , Jens Axboe , Sean Christopherson , linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2] entry/kvm: Make vCPU tasks exit to userspace when a livepatch is pending Message-ID: References: <20220503174934.2641605-1-sforshee@digitalocean.com> <20220504130753.GB8069@pathway.suse.cz> <20220504142809.GC8069@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220504142809.GC8069@pathway.suse.cz> Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On Wed, May 04, 2022 at 04:28:09PM +0200, Petr Mladek wrote: > On Wed 2022-05-04 08:50:22, Seth Forshee wrote: > > On Wed, May 04, 2022 at 03:07:53PM +0200, Petr Mladek wrote: > > > On Tue 2022-05-03 12:49:34, Seth Forshee wrote: > > > > A task can be livepatched only when it is sleeping or it exits to > > > > userspace. This may happen infrequently for a heavily loaded vCPU task, > > > > leading to livepatch transition failures. > > > > > > The problem was solved by sending a fake signal, see the commit > > > 0b3d52790e1cfd6b80b826 ("livepatch: Remove signal sysfs attribute"). > > > It was achieved by calling signal_wake_up(). It set TIF_SIGPENDING > > > and woke the task. It interrupted the syscall and the task was > > > transitioned when leaving to the userspace. > > > > > > signal_wake_up() was later replaced by set_notify_signal(), > > > see the commit 8df1947c71ee53c7e21 ("livepatch: Replace > > > the fake signal sending with TIF_NOTIFY_SIGNAL infrastructure"). > > > The difference is that set_notify_signal() uses TIF_NOTIFY_SIGNAL > > > instead of TIF_SIGPENDING. > > > > > > The effect is the same when running on a real hardware. The syscall > > > gets interrupted and exit_to_user_mode_loop() is called where > > > the livepatch state is updated (task migrated). > > > > > > But it works a different way in kvm where the task works are > > > called in the guest mode and the task does not return into > > > the user space in the host mode. > > > > > > --- a/kernel/entry/kvm.c > > > > +++ b/kernel/entry/kvm.c > > > > @@ -14,7 +14,12 @@ static int xfer_to_guest_mode_work(struct kvm_vcpu *vcpu, unsigned long ti_work) > > > > task_work_run(); > > > > } > > > > > > > > - if (ti_work & _TIF_SIGPENDING) { > > > > + /* > > > > + * When a livepatch is pending, force an exit to userspace > > > > + * as though a signal is pending to allow the task to be > > > > + * patched. > > > > + */ > > > > + if (ti_work & (_TIF_SIGPENDING | _TIF_PATCH_PENDING)) { > > > > kvm_handle_signal_exit(vcpu); > > Another problem. Is it safe to call kvm_handle_signal_exit(vcpu) > for kthreads? > > kthreads have _TIF_PATCH_PENDING when they need the livepatch transition. > But kthreads never leave kernel so we do not send the fake signal > signals to them. xfer_to_guest_mode_handle_work() should only be getting called on user threads running ioctl(KVM_RUN). > > > > > return -EINTR; > > > > } > > > > > > Does xfer_to_guest_mode_work() interrupts the syscall running > > > on the guest? > > > > xfer_to_guest_mode_work() is called as part of a loop to execute kvm > > guests (for example, on x86 see vcpu_run() in arch/x86/kvm/x86.c). When > > guest execution is interrupted (in the livepatch case it is interrupted > > when set_notify_signal() is called for the vCPU task) > > xfer_to_guest_mode_work() is called if there is pending work, and if it > > returns non-zero the loop does not immediately re-enter guest execution > > but instead returns to userspace. > > Thanks for the detailed explanation. > > > > > If "yes" then we do not need to call kvm_handle_signal_exit(vcpu). > > > It will be enough to call: > > > > > > if (ti_work & _TIF_PATCH_PENDING) > > > klp_update_patch_state(current); > > > > What if the task's call stack contains a function being patched? > > We do not need to check the stack when the syscall gets restarted. > The task might be transitioned only when the syscall gets restarted. I see. Thanks! > > > If "no" then I do not understand why TIF_NOTIFY_SIGNAL interrupts > > > the syscall on the real hardware and not in kvm. > > > > It does interrupt, but xfer_to_guest_mode_handle_work() concludes it's > > not necessary to return to userspace and resumes guest execution. > > In this case, we should revert the commit 8df1947c71ee53c7e21 > ("livepatch: Replace the fake signal sending with TIF_NOTIFY_SIGNAL > infrastructure"). The flag TIF_NOTIFY_SIGNAL clearly does not guarantee > restarting the syscall or exiting to the user space with -EINTR. > > It should solve this problem. And it looks like a cleaner solution > to me. It looks like that should fix the issue. I'll test to confirm. Thanks, Seth