linux-kernel - Re: [Xen-devel] [PATCH] Revert "swiotlb: remove SWIOTLB_MAP

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Date:   Wed, 13 Mar 2019 19:32:54 +0100
From:   Christoph Hellwig <hch@....de>
To:     Julien Grall <julien.grall@....com>
Cc:     Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
        Robin Murphy <robin.murphy@....com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [Xen-devel] [PATCH] Revert "swiotlb: remove SWIOTLB_MAP_ERROR"

On Fri, Mar 08, 2019 at 05:25:57PM +0000, Julien Grall wrote:
> In the common case, Dom0 also contains the PV backend drivers. Those 
> drivers may directly use the guest buffer in the DMA request (so a copy is 
> avoided). To avoid using a bounce buffer too much, xen-swiotlb will find 
> the host physical address associated to the guest buffer and will use it to 
> compute the DMA address.
>
> While Dom0 kernel may only deal with 32-bit physical address, the 
> hypervisor can still deal with up to 40-bit physical address. This means 
> the guest memory can be allocated above the 4GB threshold. Hence why the 
> dma_addr_t is always 64-bit with CONFIG_XEN=y.

This at least makes some sense.  But is it really so much better to
avoid having a 64-bit phys_addr_t?