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

[Brian Gerst <brgerst@xxxxxxxxx>]

On Tue, Jul 11, 2017 at 11:02 AM, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> On 7/10/2017 11:58 PM, Brian Gerst wrote:
>>
>> 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().
>
>
> Good catch.  I'll update the patch to include the removal of the ISA
> checks in the iounmap() path as well.

I now think it should be kept but also emit a warning, at least for
the short term.  There is bad code out there (vga16fb for example)
that calls iounmap() blindly without calling ioremap() first.  We
don't want to actually follow through with the unmap on the linear
mapping.

--
Brian Gerst

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