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Subject: Re: [GIT PULL] dma-mapping fix for Linux 5.14
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        iommu <iommu@lists.linux-foundation.org>,
        Stefano Stabellini <sstabellini@kernel.org>
References: <YPz+qQ6dbZVDbMwu@infradead.org>
 <CAHk-=wi2OMmUkZFdQ0=uYmGeC3sv3eYw-p1=d51pJS-XVKaM2A@mail.gmail.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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On 7/25/21 12:50 PM, Linus Torvalds wrote:
> On Sat, Jul 24, 2021 at 11:03 PM Christoph Hellwig <hch@infradead.org> wrote:
>
>>   - handle vmalloc addresses in dma_common_{mmap,get_sgtable}
>>     (Roman Skakun)
> I've pulled this, but my reaction is that we've tried to avoid this in
> the past. Why is Xen using vmalloc'ed addresses and passing those in
> to the dma mapping routines?
>
> It *smells* to me like a Xen-swiotlb bug, and it would have been
> better to try to fix it there. Was that just too painful?


Stefano will probably know better but this appears to have something to do with how Pi (and possibly more ARM systems?) manage DMA memory: https://lore.kernel.org/xen-devel/CADz_WD5Ln7Pe1WAFp73d2Mz9wxspzTE3WgAJusp5S8LX4=83Bw@mail.gmail.com/.


-boris