linux-kernel - RE: [RFC 1/1] swiotlb: Reduce calls to swiotlb_find

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Date: Thu, 27 Jun 2024 15:04:35 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Petr Tesařík <petr@...arici.cz>,
	"mhkelley58@...il.com" <mhkelley58@...il.com>
CC: "robin.murphy@....com" <robin.murphy@....com>, "joro@...tes.org"
	<joro@...tes.org>, "will@...nel.org" <will@...nel.org>, "jgross@...e.com"
	<jgross@...e.com>, "sstabellini@...nel.org" <sstabellini@...nel.org>,
	"oleksandr_tyshchenko@...m.com" <oleksandr_tyshchenko@...m.com>, "hch@....de"
	<hch@....de>, "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: RE: [RFC 1/1] swiotlb: Reduce calls to swiotlb_find_pool()

From: Petr Tesařík <petr@...arici.cz> Sent: Thursday, June 27, 2024 12:21 AM

[...]

> > @@ -187,10 +169,13 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
> >  	 * This barrier pairs with smp_mb() in swiotlb_find_slots().
> >  	 */
> >  	smp_rmb();
> > -	return READ_ONCE(dev->dma_uses_io_tlb) &&
> > -		swiotlb_find_pool(dev, paddr);
> > +	if (READ_ONCE(dev->dma_uses_io_tlb))
> > +		return swiotlb_find_pool(dev, paddr);
> > +	return NULL;
> >  #else
> > -	return paddr >= mem->defpool.start && paddr < mem->defpool.end;
> > +	if (paddr >= mem->defpool.start && paddr < mem->defpool.end)
> > +		return &mem->defpool;
> 
> Why are we open-coding swiotlb_find_pool() here? It does not make a
> difference now, but if swiotlb_find_pool() were to change, both places
> would have to be updated.
> 
> Does it save a reload from dev->dma_io_tlb_mem? IOW is the compiler
> unable to optimize it away?
> 
> What about this (functionally identical) variant:
> 
> #ifdef CONFIG_SWIOTLB_DYNAMIC
> 	smp_rmb();
> 	if (!READ_ONCE(dev->dma_uses_io_tlb))
> 		return NULL;
> #else
> 	if (paddr < mem->defpool.start || paddr >= mem->defpool.end);
> 		return NULL;
> #endif
> 
> 	return swiotlb_find_pool(dev, paddr);
> 

Yeah, I see your point. I'll try this and see what the generated code
looks like. It might take me a couple of days to get to it.

Michael