linux-kernel - Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local

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Date:   Fri, 17 Jun 2022 01:44:30 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     kwankhede@...dia.com, corbet@....net, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, farman@...ux.ibm.com,
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        kevin.tian@...el.com, jchrist@...ux.ibm.com, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

On Thu, Jun 16, 2022 at 04:52:11PM -0700, Nicolin Chen wrote:
> The pinned PFN list returned from vfio_pin_pages() is simply converted
> using page_to_pfn() without protection, so direct access via memcpy()
> will crash on S390 if the PFN is an IO PFN. Instead, the pages should
> be touched using kmap_local_page().

I don't see how this helps.  kmap_local_page only works for either
pages in the kernel direct map or highmem, but not for memory that needs
to be ioremapped.  And there is no highmem on s390.