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Subject: Re: [PATCH V5 09/14] powerpc/vas: Update CSB and notify process
 for fault CRBs
From: Haren Myneni <haren@linux.ibm.com>
To: Michael Neuling <mikey@neuling.org>
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Date: Sun, 09 Feb 2020 21:12:49 -0800
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Mikey, Thanks for your review comments.

On Fri, 2020-02-07 at 16:46 +1100, Michael Neuling wrote:
> On Wed, 2020-01-22 at 00:17 -0800, Haren Myneni wrote:
> > For each fault CRB, update fault address in CRB (fault_storage_addr)
> > and translation error status in CSB so that user space can touch the
> > fault address and resend the request. If the user space passed invalid
> > CSB address send signal to process with SIGSEGV.
> > 
> > Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
> > Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> > ---
> >  arch/powerpc/platforms/powernv/vas-fault.c | 116
> > +++++++++++++++++++++++++++++
> >  1 file changed, 116 insertions(+)
> > 
> > diff --git a/arch/powerpc/platforms/powernv/vas-fault.c
> > b/arch/powerpc/platforms/powernv/vas-fault.c
> > index 5c2cada..2cfab0c 100644
> > --- a/arch/powerpc/platforms/powernv/vas-fault.c
> > +++ b/arch/powerpc/platforms/powernv/vas-fault.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/slab.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/kthread.h>
> > +#include <linux/sched/signal.h>
> >  #include <linux/mmu_context.h>
> >  #include <asm/icswx.h>
> >  
> > @@ -26,6 +27,120 @@
> >  #define VAS_FAULT_WIN_FIFO_SIZE	(4 << 20)
> >  
> >  /*
> > + * Update the CSB to indicate a translation error.
> > + *
> > + * If the fault is in the CSB address itself or if we are unable to
> > + * update the CSB, send a signal to the process, because we have no
> > + * other way of notifying the user process.
> > + *
> > + * Remaining settings in the CSB are based on wait_for_csb() of
> > + * NX-GZIP.
> > + */
> > +static void update_csb(struct vas_window *window,
> > +			struct coprocessor_request_block *crb)
> > +{
> > +	int rc;
> > +	struct pid *pid;
> > +	void __user *csb_addr;
> > +	struct task_struct *tsk;
> > +	struct kernel_siginfo info;
> > +	struct coprocessor_status_block csb;
> > +
> > +	/*
> > +	 * NX user space windows can not be opened for task->mm=NULL
> > +	 * and faults will not be generated for kernel requests.
> > +	 */
> > +	if (!window->mm || !window->user_win)
> > +		return;
> > +
> > +	csb_addr = (void *)be64_to_cpu(crb->csb_addr);
> > +
> > +	csb.cc = CSB_CC_TRANSLATION;
> > +	csb.ce = CSB_CE_TERMINATION;
> > +	csb.cs = 0;
> > +	csb.count = 0;
> > +
> > +	/*
> > +	 * Returns the fault address in CPU format since it is passed with
> > +	 * signal. But if the user space expects BE format, need changes.
> > +	 * i.e either kernel (here) or user should convert to CPU format.
> > +	 * Not both!
> > +	 */
> > +	csb.address = be64_to_cpu(crb->stamp.nx.fault_storage_addr);
> 
> This looks wrong and I don't understand the comment. You need to convert this
> back to be64 to write it to csb.address. ie.
> 
>   csb.address = cpu_to_be64(be64_to_cpu(crb->stamp.nx.fault_storage_addr));
> 
> Which I think you can just avoid the endian conversion all together.

NX pastes fault CRB in big-endian, so passing this address in CPU format
to user space, otherwise the library has to convert. 

What is the standard way for passing to user space? 

> 
> > +	csb.flags = 0;
> > +
> > +	pid = window->pid;
> > +	tsk = get_pid_task(pid, PIDTYPE_PID);
> > +	/*
> > +	 * Send window will be closed after processing all NX requests
> > +	 * and process exits after closing all windows. In multi-thread
> > +	 * applications, thread may not exists, but does not close FD
> > +	 * (means send window) upon exit. Parent thread (tgid) can use
> > +	 * and close the window later.
> > +	 * pid and mm references are taken when window is opened by
> > +	 * process (pid). So tgid is used only when child thread opens
> > +	 * a window and exits without closing it in multithread tasks.
> > +	 */
> > +	if (!tsk) {
> > +		pid = window->tgid;
> > +		tsk = get_pid_task(pid, PIDTYPE_PID);
> > +		/*
> > +		 * Parent thread will be closing window during its exit.
> > +		 * So should not get here.
> > +		 */
> > +		if (!tsk)
> > +			return;
> > +	}
> > +
> > +	/* Return if the task is exiting. */
> > +	if (tsk->flags & PF_EXITING) {
> > +		put_task_struct(tsk);
> > +		return;
> > +	}
> > +
> > +	use_mm(window->mm);
> > +	rc = copy_to_user(csb_addr, &csb, sizeof(csb));
> > +	/*
> > +	 * User space polls on csb.flags (first byte). So add barrier
> > +	 * then copy first byte with csb flags update.
> > +	 */
> > +	smp_mb();
> > +	if (!rc) {
> > +		csb.flags = CSB_V;
> > +		rc = copy_to_user(csb_addr, &csb, sizeof(u8));
> > +	}
> > +	unuse_mm(window->mm);
> > +	put_task_struct(tsk);
> > +
> > +	/* Success */
> > +	if (!rc)
> > +		return;
> > +
> > +	pr_err("Invalid CSB address 0x%p signalling pid(%d)\n",
> > +			csb_addr, pid_vnr(pid));
> 
> This is a userspace error, not a kernel error. This should not be a pr_err().
> 
> Userspace could spam the console with this.

Will change it to pr_debug/info. Added pr_err() during development and
missed to remove. 
> 
> > +
> > +	clear_siginfo(&info);
> > +	info.si_signo = SIGSEGV;
> > +	info.si_errno = EFAULT;
> > +	info.si_code = SEGV_MAPERR;
> > +	info.si_addr = csb_addr;
> > +
> > +	/*
> > +	 * process will be polling on csb.flags after request is sent to
> > +	 * NX. So generally CSB update should not fail except when an
> > +	 * application does not follow the process properly. So an error
> > +	 * message will be displayed and leave it to user space whether
> > +	 * to ignore or handle this signal.
> > +	 */
> > +	rcu_read_lock();
> > +	rc = kill_pid_info(SIGSEGV, &info, pid);
> > +	rcu_read_unlock();
> 
> why the rcu_read_un/lock() here?

Used same as in kill_proc_info()/kill_something_info()
> 
> > +
> > +	pr_devel("%s(): pid %d kill_proc_info() rc %d\n", __func__,
> > +			pid_vnr(pid), rc);
> > +}
> > +
> > +/*
> >   * Process CRBs that we receive on the fault window.
> >   */
> >  irqreturn_t vas_fault_handler(int irq, void *data)
> > @@ -104,6 +219,7 @@ irqreturn_t vas_fault_handler(int irq, void *data)
> >  			return IRQ_HANDLED;
> >  		}
> >  
> > +		update_csb(window, crb);
> >  	} while (true);
> >  
> >  	return IRQ_HANDLED;
>