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

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Message-ID: <Yqz5IHqDrLNW0htv@Asurada-Nvidia>
Date:   Fri, 17 Jun 2022 14:58:56 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     <kwankhede@...dia.com>, <corbet@....net>, <hca@...ux.ibm.com>,
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Subject: Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

On Fri, Jun 17, 2022 at 01:44:30AM -0700, Christoph Hellwig wrote:
> 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.

Oh..I will discuss with Jason once he's back, to see if we should
simply drop this change. Thanks!