From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A9EA910A1F for ; Wed, 23 Apr 2025 10:54:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745405677; cv=none; b=qshoQQHIjqmuINcdInQdkeBSNtOsacmzSmkjpHPIRpBAvl1iuFzQfPXsPL/ibWMgTI7wm4Xp+oxPIFYabSKs6n+UsWWyWe2BRjDYZFk4inoZH99cc8n2iXArz4N0vGQwM/90EpGCh2Fy/gcPidQa1/gDe6rg42aA6mB/5FdpK8U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1745405677; c=relaxed/simple; bh=aHDn6ge4cRBqqSuItsEuM+6K5lzg1tcbbcPgl6BmAqU=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=q51Fi9Q9YymWBFnl13fyuZrGUmE3cMlZ/GD6E+6QimUCB8FbjAHXyqZMUcmxv0MQ2fZ7y4h5pSmbs08iO5/jOuBraEjdIl6tKKvmbbgh/oRMSEej2jen++iC1uFc4GOx+NRVlrxHBLBnvPB32sy/iGMaJPR+rbl8ofa6amAknPs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=X4S/NzTs; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="X4S/NzTs" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DC334C4CEEB; Wed, 23 Apr 2025 10:54:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1745405677; bh=aHDn6ge4cRBqqSuItsEuM+6K5lzg1tcbbcPgl6BmAqU=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=X4S/NzTsdED0aV243qNsPRLS3sb2IJy5mVDOiVNhWW/9geXHddeB68yl/i/1znY7E 9u1ru6W3l1W48rO1Veqrkubu6gZirHiSqL5TZSGuq8LP8Ka2wZ3jQWWPkRgVuwMt/d jiJ8wOtPZ2HvJbTZp4qcSn8MQuhsqM7GsWiQ1mVVJT9M1CltQF6IomNZFLHFuLeKa+ cVUPIr3TeOWyJr5Jxsj0P53f4jVjAPZYJ810Bav1fiSA5Kh1DVBrINDHLAKDv8A3MV 8rjDSnxoXnR/830yXaKPr6iNrVeOA9v4ZCQXeC4T4ZGMsiwVKDsVEIvX1JonlSC8db EofnD9ZILsu+g== X-Mailer: emacs 30.1 (via feedmail 11-beta-1 Q) From: Aneesh Kumar K.V To: Will Deacon Cc: kvm@vger.kernel.org, Suzuki K Poulose , Steven Price , Julien Thierry , Alexandru Elisei Subject: Re: [PATCH kvmtool v2 1/2] cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly In-Reply-To: <20250417120701.GA12773@willie-the-truck> References: <20250224091000.3925918-1-aneesh.kumar@kernel.org> <20250417120701.GA12773@willie-the-truck> Date: Wed, 23 Apr 2025 16:23:38 +0530 Message-ID: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Will Deacon writes: > On Mon, Feb 24, 2025 at 02:39:59PM +0530, Aneesh Kumar K.V (Arm) wrote: >> The return value for kernel VM exit handlers is confusing and has led to >> errors in different kernel exit handlers. A return value of 0 indicates >> a return to the VMM, whereas a return value of 1 indicates resuming >> execution in the guest. Some handlers mistakenly return 0 to force a >> return to the guest. >> >> This worked in kvmtool because the exit_reason defaulted to >> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit >> reason. However, forcing a VMM exit with error on KVM_EXIT_UNKNOWN >> exit_reson would help catch these bugs early. >> >> Signed-off-by: Aneesh Kumar K.V (Arm) >> --- >> kvm-cpu.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kvm-cpu.c b/kvm-cpu.c >> index f66dcd07220c..7c62bfc56679 100644 >> --- a/kvm-cpu.c >> +++ b/kvm-cpu.c >> @@ -170,7 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu) >> >> switch (cpu->kvm_run->exit_reason) { >> case KVM_EXIT_UNKNOWN: >> - break; >> + goto panic_kvm; >> case KVM_EXIT_DEBUG: >> kvm_cpu__show_registers(cpu); >> kvm_cpu__show_code(cpu); >> -- >> 2.43.0 > > This breaks SMP boot on my x86 machine: > > # ./lkvm run > ... > [ 0.628472] smp: Bringing up secondary CPUs ... > [ 0.630401] smpboot: x86: Booting SMP configuration: > Error: KVM exit reason: 0 ("KVM_EXIT_UNKNOWN") > Error: KVM exit code: 0 > Turns out we should handle EINTR and EAGAIN as special such that we do an retry of KVM_RUN ioctl without checking the exit_reason. I can send a v3 if you are ok with the change below. @@ -16,7 +16,7 @@ void kvm_cpu__delete(struct kvm_cpu *vcpu); void kvm_cpu__reset_vcpu(struct kvm_cpu *vcpu); void kvm_cpu__setup_cpuid(struct kvm_cpu *vcpu); void kvm_cpu__enable_singlestep(struct kvm_cpu *vcpu); -void kvm_cpu__run(struct kvm_cpu *vcpu); +int kvm_cpu__run(struct kvm_cpu *vcpu); int kvm_cpu__start(struct kvm_cpu *cpu); bool kvm_cpu__handle_exit(struct kvm_cpu *vcpu); int kvm_cpu__get_endianness(struct kvm_cpu *vcpu); modified kvm-cpu.c @@ -35,27 +35,32 @@ void kvm_cpu__enable_singlestep(struct kvm_cpu *vcpu) pr_warning("KVM_SET_GUEST_DEBUG failed"); } -void kvm_cpu__run(struct kvm_cpu *vcpu) +/* + * return value -1 if we need to call the kvm_cpu__run again without checking + * exit_reason. return value 0 results in taking action based on exit_reason. + */ +int kvm_cpu__run(struct kvm_cpu *vcpu) { int err; if (!vcpu->is_running) - return; + return -1; err = ioctl(vcpu->vcpu_fd, KVM_RUN, 0); if (err < 0) { switch (errno) { case EINTR: case EAGAIN: - return; + return -1; case EFAULT: if (vcpu->kvm_run->exit_reason == KVM_EXIT_MEMORY_FAULT) - return; + return 0; /* faullthrough */ default: die_perror("KVM_RUN failed"); } } + return 0; } static void kvm_cpu_signal_handler(int signum) @@ -179,11 +184,13 @@ int kvm_cpu__start(struct kvm_cpu *cpu) if (cpu->task) kvm_cpu__run_task(cpu); - kvm_cpu__run(cpu); + if (kvm_cpu__run(cpu) == -1) + /* retry without an exit_reason check */ + continue; -aneesh