Re: [PATCH v4 4/9] powerpc/vas: Return paste instruction failure if no active window

[Haren Myneni <haren@linux.ibm.com>]

On Wed, 2022-02-23 at 17:05 +1000, Nicholas Piggin wrote:
> Excerpts from Haren Myneni's message of February 20, 2022 5:58 am:
> > The VAS window may not be active if the system looses credits and
> > the NX generates page fault when it receives request on unmap
> > paste address.
> > 
> > The kernel handles the fault by remap new paste address if the
> > window is active again, Otherwise return the paste instruction
> > failure if the executed instruction that caused the fault was
> > a paste.
> 
> Looks good, thanks for fixin the SIGBUS thing, was that my
> fault? I vaguely remember writing some of this patch :P

Thanks for your reviews on all patches. 

No, it was my fault not handling the -EGAIN error. 

> 
> Thanks,
> Nick
> 
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> > ---
> >  arch/powerpc/include/asm/ppc-opcode.h   |  2 +
> >  arch/powerpc/platforms/book3s/vas-api.c | 55
> > ++++++++++++++++++++++++-
> >  2 files changed, 56 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/include/asm/ppc-opcode.h
> > b/arch/powerpc/include/asm/ppc-opcode.h
> > index 9675303b724e..82f1f0041c6f 100644
> > --- a/arch/powerpc/include/asm/ppc-opcode.h
> > +++ b/arch/powerpc/include/asm/ppc-opcode.h
> > @@ -262,6 +262,8 @@
> >  #define PPC_INST_MFSPR_PVR		0x7c1f42a6
> >  #define PPC_INST_MFSPR_PVR_MASK		0xfc1ffffe
> >  #define PPC_INST_MTMSRD			0x7c000164
> > +#define PPC_INST_PASTE			0x7c20070d
> > +#define PPC_INST_PASTE_MASK		0xfc2007ff
> >  #define PPC_INST_POPCNTB		0x7c0000f4
> >  #define PPC_INST_POPCNTB_MASK		0xfc0007fe
> >  #define PPC_INST_RFEBB			0x4c000124
> > diff --git a/arch/powerpc/platforms/book3s/vas-api.c
> > b/arch/powerpc/platforms/book3s/vas-api.c
> > index f359e7b2bf90..f3e421511ea6 100644
> > --- a/arch/powerpc/platforms/book3s/vas-api.c
> > +++ b/arch/powerpc/platforms/book3s/vas-api.c
> > @@ -351,6 +351,41 @@ static int coproc_release(struct inode *inode,
> > struct file *fp)
> >  	return 0;
> >  }
> >  
> > +/*
> > + * If the executed instruction that caused the fault was a paste,
> > then
> > + * clear regs CR0[EQ], advance NIP, and return 0. Else return
> > error code.
> > + */
> > +static int do_fail_paste(void)
> > +{
> > +	struct pt_regs *regs = current->thread.regs;
> > +	u32 instword;
> > +
> > +	if (WARN_ON_ONCE(!regs))
> > +		return -EINVAL;
> > +
> > +	if (WARN_ON_ONCE(!user_mode(regs)))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * If we couldn't translate the instruction, the driver should
> > +	 * return success without handling the fault, it will be
> > retried
> > +	 * or the instruction fetch will fault.
> > +	 */
> > +	if (get_user(instword, (u32 __user *)(regs->nip)))
> > +		return -EAGAIN;
> > +
> > +	/*
> > +	 * Not a paste instruction, driver may fail the fault.
> > +	 */
> > +	if ((instword & PPC_INST_PASTE_MASK) != PPC_INST_PASTE)
> > +		return -ENOENT;
> > +
> > +	regs->ccr &= ~0xe0000000;	/* Clear CR0[0-2] to fail paste */
> > +	regs_add_return_ip(regs, 4);	/* Emulate the paste */
> > +
> > +	return 0;
> > +}
> > +
> >  /*
> >   * This fault handler is invoked when the core generates page
> > fault on
> >   * the paste address. Happens if the kernel closes window in
> > hypervisor
> > @@ -408,9 +443,27 @@ static vm_fault_t vas_mmap_fault(struct
> > vm_fault *vmf)
> >  	}
> >  	mutex_unlock(&txwin->task_ref.mmap_mutex);
> >  
> > -	return VM_FAULT_SIGBUS;
> > +	/*
> > +	 * Received this fault due to closing the actual window.
> > +	 * It can happen during migration or lost credits.
> > +	 * Since no mapping, return the paste instruction failure
> > +	 * to the user space.
> > +	 */
> > +	ret = do_fail_paste();
> > +	/*
> > +	 * The user space can retry several times until success (needed
> > +	 * for migration) or should fallback to SW compression or
> > +	 * manage with the existing open windows if available.
> > +	 * Looking at sysfs interface, it can determine whether these
> > +	 * failures are coming during migration or core removal:
> > +	 * nr_used_credits > nr_total_credits when lost credits
> > +	 */
> > +	if (!ret || (ret == -EAGAIN))
> > +		return VM_FAULT_NOPAGE;
> >  
> > +	return VM_FAULT_SIGBUS;
> >  }
> > +
> >  static const struct vm_operations_struct vas_vm_ops = {
> >  	.fault = vas_mmap_fault,
> >  };
> > -- 
> > 2.27.0
> > 
> > 
> >