Re: [PATCH 08/11] ACPICA: Tables: Back port acpi_get_table_with_size() and early_acpi_os_unmap_memory() from Linux kernel

[Dan Williams]

On Thu, Dec 8, 2016 at 5:18 AM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> On Thu, Dec 8, 2016 at 2:11 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> On Tue, Nov 29, 2016 at 11:21 PM, Lv Zheng <lv.zheng@xxxxxxxxx> wrote:
>>> ACPICA commit cac6790954d4d752a083e6122220b8a22febcd07
>>>
>>> This patch back ports Linux acpi_get_table_with_size() and
>>> early_acpi_os_unmap_memory() into ACPICA upstream to reduce divergences.
>>>
>>> The 2 APIs are used by Linux as table management APIs for long time, it
>>> contains a hidden logic that during the early stage, the mapped tables
>>> should be unmapped before the early stage ends.
>>>
>>> During the early stage, tables are handled by the following sequence:
>>>  acpi_get_table_with_size();
>>>  parse the table
>>>  early_acpi_os_unmap_memory();
>>> During the late stage, tables are handled by the following sequence:
>>>  acpi_get_table();
>>>  parse the table
>>> Linux uses acpi_gbl_permanent_mmap to distinguish the early stage and the
>>> late stage.
>>>
>>> The reasoning of introducing acpi_get_table_with_size() is: ACPICA will
>>> remember the early mapped pointer in acpi_get_table() and Linux isn't able to
>>> prevent ACPICA from using the wrong early mapped pointer during the late
>>> stage as there is no API provided from ACPICA to be an inverse of
>>> acpi_get_table() to forget the early mapped pointer.
>>>
>>> But how ACPICA can work with the early/late stage requirement? Inside of
>>> ACPICA, tables are ensured to be remained in "INSTALLED" state during the
>>> early stage, and they are carefully not transitioned to "VALIDATED" state
>>> until the late stage. So the same logic is in fact implemented inside of
>>> ACPICA in a different way. The gap is only that the feature is not provided
>>> to the OSPMs in an accessible external API style.
>>>
>>> It then is possible to fix the gap by providing an inverse of
>>> acpi_get_table() from ACPICA, so that the two Linux sequences can be
>>> combined:
>>>  acpi_get_table();
>>>  parse the table
>>>  acpi_put_table();
>>> In order to work easier with the current Linux code, acpi_get_table() and
>>> acpi_put_table() is implemented in a usage counting based style:
>>>  1. When the usage count of the table is increased from 0 to 1, table is
>>>     mapped and .Pointer is set with the mapping address (VALIDATED);
>>>  2. When the usage count of the table is decreased from 1 to 0, .Pointer
>>>     is unset and the mapping address is unmapped (INVALIDATED).
>>> So that we can deploy the new APIs to Linux with minimal effort by just
>>> invoking acpi_get_table() in acpi_get_table_with_size() and invoking
>>> acpi_put_table() in early_acpi_os_unmap_memory(). Lv Zheng.
>>>
>>> Link: https://github.com/acpica/acpica/commit/cac67909
>>> Signed-off-by: Lv Zheng <lv.zheng@xxxxxxxxx>
>>> Signed-off-by: Bob Moore <robert.moore@xxxxxxxxx>
>>
>> This commit in -next (071b39575679 ACPICA: Tables: Back port
>> acpi_get_table_with_size() and early_acpi_os_unmap_memory() from Linux
>> kernel) causes a regression in my nfit/nvdimm test environment. The
>> nfit produced by QEMU no longer results in a nvdimm bus being created.
>>
>> I have not root caused it, but I'm using the following command line
>> options to create an nfit in qemu-2.6.  Reverting the commit leads
>> compile failures.
>
> Would the build problems go away if you reverted "ACPICA: Tables:
> Allow FADT to be customized with virtual address" (linux-next commit
> cf334d3174f9) in addition to it?

Yes, reverting those two commits gets me back to a functional environment:

Revert "ACPICA: Tables: Allow FADT to be customized with virtual address"
Revert "ACPICA: Tables: Back port acpi_get_table_with_size() and
early_acpi_os_un