Re: [Xen-devel] [PATCH v9 07/38] x86/mm: Remove phys_to_virt() usage in ioremap()

[Brian Gerst <brgerst@xxxxxxxxx>]

On Mon, Jul 10, 2017 at 3:50 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> On 7/8/2017 7:57 AM, Brian Gerst wrote:
>>
>> On Fri, Jul 7, 2017 at 9:39 AM, Tom Lendacky <thomas.lendacky@xxxxxxx>
>> wrote:
>>>
>>> Currently there is a check if the address being mapped is in the ISA
>>> range (is_ISA_range()), and if it is, then phys_to_virt() is used to
>>> perform the mapping. When SME is active, the default is to add pagetable
>>> mappings with the encryption bit set unless specifically overridden. The
>>> resulting pagetable mapping from phys_to_virt() will result in a mapping
>>> that has the encryption bit set. With SME, the use of ioremap() is
>>> intended to generate pagetable mappings that do not have the encryption
>>> bit set through the use of the PAGE_KERNEL_IO protection value.
>>>
>>> Rather than special case the SME scenario, remove the ISA range check and
>>> usage of phys_to_virt() and have ISA range mappings continue through the
>>> remaining ioremap() path.
>>>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>> ---
>>>   arch/x86/mm/ioremap.c |    7 +------
>>>   1 file changed, 1 insertion(+), 6 deletions(-)
>>>
>>> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
>>> index 4c1b5fd..bfc3e2d 100644
>>> --- a/arch/x86/mm/ioremap.c
>>> +++ b/arch/x86/mm/ioremap.c
>>> @@ -13,6 +13,7 @@
>>>   #include <linux/slab.h>
>>>   #include <linux/vmalloc.h>
>>>   #include <linux/mmiotrace.h>
>>> +#include <linux/mem_encrypt.h>
>>>
>>>   #include <asm/set_memory.h>
>>>   #include <asm/e820/api.h>
>>> @@ -106,12 +107,6 @@ static void __iomem
>>> *__ioremap_caller(resource_size_t phys_addr,
>>>          }
>>>
>>>          /*
>>> -        * Don't remap the low PCI/ISA area, it's always mapped..
>>> -        */
>>> -       if (is_ISA_range(phys_addr, last_addr))
>>> -               return (__force void __iomem *)phys_to_virt(phys_addr);
>>> -
>>> -       /*
>>>           * Don't allow anybody to remap normal RAM that we're using..
>>>           */
>>>          pfn      = phys_addr >> PAGE_SHIFT;
>>>
>>
>> Removing this also affects 32-bit, which is more likely to access
>> legacy devices in this range.  Put in a check for SME instead
>
>
> 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
>
> Thanks,
> Tom

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().

--
Brian Gerst

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