Re: [PATCH v2] swiotlb: fix the check whether a device has used software IO TLB

[Petr Tesařík]

On Tue, 26 Sep 2023 19:17:29 +0100
Catalin Marinas <catalin.marinas@xxxxxxx> wrote:

> On Tue, Sep 26, 2023 at 06:23:39PM +0200, Petr Tesarik wrote:
> > When CONFIG_SWIOTLB_DYNAMIC=y, devices which do not use the software IO TLB
> > can avoid swiotlb lookup. A flag is added by commit 1395706a1490 ("swiotlb:
> > search the software IO TLB only if the device makes use of it"), the flag
> > is correctly set, but it is then never checked. Add the actual check here.
> > 
> > Note that this code is an alternative to the default pool check, not an
> > additional check, because:
> > 
> > 1. swiotlb_find_pool() also searches the default pool;
> > 2. if dma_uses_io_tlb is false, the default swiotlb pool is not used.
> > 
> > Tested in a KVM guest against a QEMU RAM-backed SATA disk over virtio and
> > *not* using software IO TLB, this patch increases IOPS by approx 2% for
> > 4-way parallel I/O.
> > 
> > The write memory barrier in swiotlb_dyn_alloc() is not needed, because a
> > newly allocated pool must always be observed by swiotlb_find_slots() before
> > an address from that pool is passed to is_swiotlb_buffer().
> > 
> > Correctness was verified using the following litmus test:  
> [...]
> > Fixes: 1395706a1490 ("swiotlb: search the software IO TLB only if the device makes use of it")
> > Reported-by: Jonathan Corbet <corbet@xxxxxxx>
> > Closes: https://lore.kernel.org/linux-iommu/87a5uz3ob8.fsf@xxxxxxxxxxxx/
> > Signed-off-by: Petr Tesarik <petr@xxxxxxxxxxx>  
> 
> Thanks for the update.
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> 
> > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> > index b4536626f8ff..93b400d9be91 100644
> > --- a/include/linux/swiotlb.h
> > +++ b/include/linux/swiotlb.h
> > @@ -172,14 +172,22 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
> >  	if (!mem)
> >  		return false;
> >  
> > -	if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC)) {
> > -		/* Pairs with smp_wmb() in swiotlb_find_slots() and
> > -		 * swiotlb_dyn_alloc(), which modify the RCU lists.
> > -		 */
> > -		smp_rmb();
> > -		return swiotlb_find_pool(dev, paddr);
> > -	}
> > +#ifdef CONFIG_SWIOTLB_DYNAMIC
> > +	/* All SWIOTLB buffer addresses must have been returned by
> > +	 * swiotlb_tbl_map_single() and passed to a device driver.
> > +	 * If a SWIOTLB address is checked on another CPU, then it was
> > +	 * presumably loaded by the device driver from an unspecified private
> > +	 * data structure. Make sure that this load is ordered before reading
> > +	 * dev->dma_uses_io_tlb here and mem->pools in swiotlb_find_pool().
> > +	 *
> > +	 * This barrier pairs with smp_mb() in swiotlb_find_slots().
> > +	 */  
> 
> Nitpick. The official multi-line comment style is:
> 
> 	/*
> 	 * Text.
> 	 */
> 
> i.e. it starts with an empty /* line.

Right! I should add it to scripts/checkpatch.pl.

> > @@ -1152,9 +1149,25 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
> >  	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
> >  
> >  found:
> > -	dev->dma_uses_io_tlb = true;
> > -	/* Pairs with smp_rmb() in is_swiotlb_buffer() */
> > -	smp_wmb();
> > +	WRITE_ONCE(dev->dma_uses_io_tlb, true);
> > +
> > +	/* The general barrier orders reads and writes against a presumed store
> > +	 * of the SWIOTLB buffer address by a device driver (to a driver private
> > +	 * data structure). It serves two purposes.  
> 
> Same here.

If that's the only issue... ;-)

Petr T