From patchwork Sun Nov 30 21:49:45 2008 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Paul Mackerras X-Patchwork-Id: 11475 Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from ozlabs.org (localhost [127.0.0.1]) by ozlabs.org (Postfix) with ESMTP id C7C1ADDDF5 for ; Mon, 1 Dec 2008 08:52:18 +1100 (EST) X-Original-To: cbe-oss-dev@ozlabs.org Delivered-To: cbe-oss-dev@ozlabs.org Received: by ozlabs.org (Postfix, from userid 1003) id 254B8DDD0C; Mon, 1 Dec 2008 08:50:11 +1100 (EST) MIME-Version: 1.0 Message-ID: <18739.2681.351794.669476@cargo.ozlabs.ibm.com> Date: Mon, 1 Dec 2008 08:49:45 +1100 From: Paul Mackerras To: linuxppc-dev@ozlabs.org X-Mailer: VM 8.0.9 under Emacs 22.2.1 (i486-pc-linux-gnu) Cc: cbe-oss-dev@ozlabs.org Subject: [Cbe-oss-dev] [PATCH] powerpc: Fix system calls on Cell entered with XER.SO=1 X-BeenThere: cbe-oss-dev@ozlabs.org X-Mailman-Version: 2.1.11 Precedence: list List-Id: Discussion about Open Source Software for the Cell Broadband Engine List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: cbe-oss-dev-bounces+patchwork-incoming=ozlabs.org@ozlabs.org Errors-To: cbe-oss-dev-bounces+patchwork-incoming=ozlabs.org@ozlabs.org It turns out that on Cell, on a kernel with CONFIG_VIRT_CPU_ACCOUNTING = y, if a program sets the SO (summary overflow) bit in the XER and then does a system call, the SO bit in CR0 will be set on return regardless of whether the system call detected an error. Since CR0.SO is used as the error indication from the system call, this means that all system calls appear to fail. The reason is that the workaround for the timebase bug on Cell uses a compare instruction. With CONFIG_VIRT_CPU_ACCOUNTING = y, the ACCOUNT_CPU_USER_ENTRY macro reads the timebase, so we end up doing a compare instruction, which copies XER.SO to CR0.SO. Since we were doing this in the system call entry patch after clearing CR0.SO but before saving the CR, this meant that the saved CR image had CR0.SO set if XER.SO was set on entry. This fixes it by moving the clearing of CR0.SO to after the ACCOUNT_CPU_USER_ENTRY call in the system call entry path. Signed-off-by: Paul Mackerras Acked-by: Arnd Bergmann Acked-by: Benjamin Herrenschmidt --- Same as before, just with a comment added to explain the problem for anybody hacking on this code in future. diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index e6d5284..e0bcf93 100644 --- a/arch/powerpc/kernel/entry_64.S +++ b/arch/powerpc/kernel/entry_64.S @@ -57,12 +57,18 @@ system_call_common: beq- 1f ld r1,PACAKSAVE(r13) 1: std r10,0(r1) - crclr so std r11,_NIP(r1) std r12,_MSR(r1) std r0,GPR0(r1) std r10,GPR1(r1) ACCOUNT_CPU_USER_ENTRY(r10, r11) + /* + * This "crclr so" clears CR0.SO, which is the error indication on + * return from this system call. There must be no cmp instruction + * between it and the "mfcr r9" below, otherwise if XER.SO is set, + * CR0.SO will get set, causing all system calls to appear to fail. + */ + crclr so std r2,GPR2(r1) std r3,GPR3(r1) std r4,GPR4(r1)