linux-kernel - Re: Phyr Starter

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Date:   Thu, 20 Jan 2022 15:03:40 +0100
From:   Christoph Hellwig <hch@....de>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jason Gunthorpe <jgg@...dia.com>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Joao Martins <joao.m.martins@...cle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-rdma@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        nvdimm@...ts.linux.dev
Subject: Re: Phyr Starter

On Wed, Jan 12, 2022 at 06:37:03PM +0000, Matthew Wilcox wrote:
> But let's go further than that (which only brings us to 32 bytes per
> range).  For the systems you care about which use an identity mapping,
> and have sizeof(dma_addr_t) == sizeof(phys_addr_t), we can simply
> point the dma_range pointer to the same memory as the phyr.  We just
> have to not free it too early.  That gets us down to 16 bytes per range,
> a saving of 33%.

Even without an IOMMU the dma_addr_t can have offsets vs the actual
physical address.  Not on x86 except for a weirdo SOC, but just about
everywhere else.