linux-kernel - Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the device makes use of it

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Message-ID: <20230831125109.GA11562@lst.de>
Date:   Thu, 31 Aug 2023 14:51:09 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jonathan Corbet <corbet@....net>
Cc:     Petr Tesarik <petrtesarik@...weicloud.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Petr Tesarik <petr.tesarik.ext@...wei.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        James Seo <james@...iv.tech>,
        James Clark <james.clark@....com>,
        Kees Cook <keescook@...omium.org>,
        "moderated list:XEN HYPERVISOR ARM" <xen-devel@...ts.xenproject.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:XEN SWIOTLB SUBSYSTEM" <iommu@...ts.linux.dev>,
        "open list:SLAB ALLOCATOR" <linux-mm@...ck.org>,
        Roberto Sassu <roberto.sassu@...weicloud.com>, petr@...arici.cz
Subject: Re: [PATCH v7 9/9] swiotlb: search the software IO TLB only if the
 device makes use of it

On Wed, Aug 09, 2023 at 03:20:43PM -0600, Jonathan Corbet wrote:
> >  	spin_unlock_irqrestore(&dev->dma_io_tlb_lock, flags);
> >  
> > -	/* Pairs with smp_rmb() in swiotlb_find_pool(). */
> > -	smp_wmb();
> >  found:
> > +	dev->dma_uses_io_tlb = true;
> > +	/* Pairs with smp_rmb() in is_swiotlb_buffer() */
> > +	smp_wmb();
> > +
> 
> ...and here you set it if swiotlb is used.
> 
> But, as far as I can tell, you don't actually *use* this field anywhere.
> What am I missing?

It's very much unused.  Petr, I guess you wanted to use this in
is_swiotlb_buffer to avoid the lookup unless required.  Can you send
a follow up?