linux-kernel - [PATCH 0/1] drivers/char/mem.c: Disable encryption bit in page tables for MMIO access

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Date:   Thu, 4 Oct 2018 13:17:26 -0400
From:   James Puthukattukaran <james.puthukattukaran@...cle.com>
To:     "Lendacky, Thomas" <thomas.lendacky@....com>,
        "Singh, Brijesh" <brijesh.singh@....com>
Cc:     gregkh@...uxfoundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH 0/1] drivers/char/mem.c: Disable encryption bit in page tables
 for MMIO access

On AMD based systems, mmap'ing a PCI MMIO region does not return proper
values. This is because the mmap_mem function does not consider the fact
that IO regions are not to be encrypted. 

In the failing kernel, here's the output  --

[root@foo]# ./memaccess 0xd0000000 -t pmem -l 32
0    (0   ) : ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 
10   (16  ) : ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 


I instrumented the kernel to print out the PTE value --

Jun 20 11:46:35 bur-e1-2l-303 kernel: pfn = 0xfffffff80f0866b2, vma->pgoff = 0xd
0000, flags = 0x5044471,  prot = 0x8000800000000025
Jun 20 11:46:35 bur-e1-2l-303 kernel: pte = 0x80008000d0000235, pfn  = 0xd0000

Note that 0x8000800000000025 -- bit 47 is set. It should not be set for a 
MMIO region. 

When I disable memory encryption (mem_encrypt=off command line), things work
as they should.



[root@foo]# ./memaccess 0xd0000000 -t pmem -l 32
0    (0   ) : 20 00 00 01 40 08 00 04 f1 00 00 14 0a 00 ff 07 
10   (16  ) : 65 f6 70 02 c0 05 00 00 a0 04 00 00 0b 00 00 00 



James Puthukattukaran(1):
  drivers/char/mem.c: Disable encryption bit in page tables for MMIO
    access

 drivers/char/mem.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)