On Tue, Jul 11, 2017 at 4:35 AM, Arnd Bergmann [off-list ref] wrote:
On Tue, Jul 11, 2017 at 6:58 AM, Brian Gerst [off-list ref] wrote:
quoted
On Mon, Jul 10, 2017 at 3:50 PM, Tom Lendacky [off-list ref] wrote:
quoted
On 7/8/2017 7:57 AM, Brian Gerst wrote:
quoted
On Fri, Jul 7, 2017 at 9:39 AM, Tom Lendacky [off-list ref]
I originally had a check for SME here in a previous version of the
patch. Thomas Gleixner recommended removing the check so that the code
path was always exercised regardless of the state of SME in order to
better detect issues:
http://marc.info/?l=linux-kernel&m=149803067811436&w=2
Looking a bit closer, this shortcut doesn't set the caching
attributes. So it's probably best to get rid of it anyways. Also
note, there is a corresponding check in iounmap().
Perhaps the iounmap() check should be kept for now for safety, since
some drivers (vga16fb for example) call iounmap() blindly even if the
mapping wasn't returned from ioremap(). Maybe add a warning when an
ISA address is passed to iounmap().
Could that cause regressions if a driver relies on (write-through)
cacheable access to the VGA frame buffer RAM or an read-only
cached access to an option ROM but now gets uncached access?
Yes there could be some surprises in drivers use the normal ioremap()
call which is uncached but were expecting the default write-through
mapping.
I also tried to find out whether we can stop mapping the ISA MMIO
area into the linear mapping, but at least the VGA code uses
VGA_MAP_MEM() to get access to the same pointers. I'm pretty
sure this got copied incorrectly into most other architectures, but
it is definitely still used on x86 with vga16fb/vgacon/mdacon.
Changing VGA_MAP_MEM() to use ioremap_wt() would take care of that.
Although, looking at the mdacon/vgacon, they don't have support for
unmapping the frame buffer if they are built modular.
--
Brian Gerst
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