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[220.132.236.182]) by smtp.gmail.com with ESMTPSA id t8sm107697374pfq.31.2019.08.08.01.00.01 (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 08 Aug 2019 01:00:02 -0700 (PDT) From: Vincent Chen To: paul.walmsley@sifive.com, palmer@sifive.com, aou@eecs.berkeley.edu Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Vincent Chen Subject: [PATCH 1/2] riscv: Correct the initialized flow of FP register Date: Thu, 8 Aug 2019 15:58:40 +0800 Message-Id: <1565251121-28490-2-git-send-email-vincent.chen@sifive.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1565251121-28490-1-git-send-email-vincent.chen@sifive.com> References: <1565251121-28490-1-git-send-email-vincent.chen@sifive.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following two reasons cause FP registers are sometimes not initialized before starting the user program. 1. Currently, the FP context is initialized in flush_thread() function and we expect these initial values to be restored to FP register when doing FP context switch. However, the FP context switch only occurs in switch_to function. Hence, if this process does not be scheduled out and scheduled in before entering the user space, the FP registers have no chance to initialize. 2. In flush_thread(), the state of reg->sstatus.FS inherits from the parent. Hence, the state of reg->sstatus.FS may be dirty. If this process is scheduled out during flush_thread() and initializing the FP register, the fstate_save() in switch_to will corrupt the FP context which has been initialized until flush_thread(). To solve the 1st case, the initialization of the FP register will be completed in start_thread(). It makes sure all FP registers are initialized before starting the user program. For the 2nd case, the state of reg->sstatus.FS in start_thread will be set to SR_FS_OFF to prevent this process from corrupting FP context in doing context save. The FP state is set to SR_FS_INITIAL in start_trhead(). Tested on both QEMU and HiFive Unleashed using BBL + Linux. Signed-off-by: Vincent Chen --- arch/riscv/include/asm/switch_to.h | 6 ++++++ arch/riscv/kernel/process.c | 13 +++++++++++-- 2 files changed, 17 insertions(+), 2 deletions(-) diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h index 853b65e..d5fe573 100644 --- a/arch/riscv/include/asm/switch_to.h +++ b/arch/riscv/include/asm/switch_to.h @@ -19,6 +19,12 @@ static inline void __fstate_clean(struct pt_regs *regs) regs->sstatus |= (regs->sstatus & ~(SR_FS)) | SR_FS_CLEAN; } +static inline void fstate_off(struct task_struct *task, + struct pt_regs *regs) +{ + regs->sstatus = (regs->sstatus & ~(SR_FS)) | SR_FS_OFF; +} + static inline void fstate_save(struct task_struct *task, struct pt_regs *regs) { diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index f23794b..e3077ee 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -64,8 +64,16 @@ void start_thread(struct pt_regs *regs, unsigned long pc, unsigned long sp) { regs->sstatus = SR_SPIE; - if (has_fpu) + if (has_fpu) { regs->sstatus |= SR_FS_INITIAL; +#ifdef CONFIG_FPU + /* + * Restore the initial value to the FP register + * before starting the user program. + */ + fstate_restore(current, regs); +#endif + } regs->sepc = pc; regs->sp = sp; set_fs(USER_DS); @@ -75,10 +83,11 @@ void flush_thread(void) { #ifdef CONFIG_FPU /* - * Reset FPU context + * Reset FPU state and context * frm: round to nearest, ties to even (IEEE default) * fflags: accrued exceptions cleared */ + fstate_off(current, task_pt_regs(current)); memset(¤t->thread.fstate, 0, sizeof(current->thread.fstate)); #endif } -- 2.7.4