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Subject: Re: [PATCH v2] Revert "ACPI: Add memory semantics to
 acpi_os_map_memory()"
To:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>
CC:     Mark Kettenis <mark.kettenis@xs4all.nl>,
        Jia He <justin.he@arm.com>,
        "Harb Abdulhamid" <harb@amperecomputing.com>,
        Will Deacon <will@kernel.org>, "Len Brown" <lenb@kernel.org>,
        Robert Moore <robert.moore@intel.com>,
        Erik Kaneda <erik.kaneda@intel.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "ACPI Devel Maling List" <linux-acpi@vger.kernel.org>,
        "open list:ACPI COMPONENT ARCHITECTURE (ACPICA)" <devel@acpica.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Hanjun Guo <guohanjun@huawei.com>,
        Catalin Marinas <catalin.marinas@arm.com>
References: <20210910122820.26886-1-justin.he@arm.com>
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From:   "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Organization: Intel Technology Poland Sp. z o. o., KRS 101882, ul. Slowackiego
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Date:   Tue, 28 Sep 2021 19:26:52 +0200
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On 9/24/2021 11:04 AM, Lorenzo Pieralisi wrote:
> On Thu, Sep 23, 2021 at 02:54:52PM +0200, Rafael J. Wysocki wrote:
>> On Thu, Sep 23, 2021 at 2:26 PM Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
>>>> From: "Rafael J. Wysocki" <rafael@kernel.org>
>>>> Date: Thu, 23 Sep 2021 13:05:05 +0200
>>>>
>>>> On Thu, Sep 23, 2021 at 11:40 AM Lorenzo Pieralisi
>>>> <lorenzo.pieralisi@arm.com> wrote:
>>>>> On Thu, Sep 23, 2021 at 01:09:58AM +0200, Mark Kettenis wrote:
>>>>>>> Date: Wed, 22 Sep 2021 17:33:36 +0100
>>>>>>> From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
>>>>>>>
>>>>>>> On Fri, Sep 10, 2021 at 10:32:23PM +0800, Jia He wrote:
>>>>>>>> This reverts commit 437b38c51162f8b87beb28a833c4d5dc85fa864e.
>>>>>>>>
>>>>>>>> After this commit, a boot panic is alway hit on an Ampere EMAG server
>>>>>>>> with call trace as follows:
>>>>>>>>   Internal error: synchronous external abort: 96000410 [#1] SMP
>>>>>>>>   Modules linked in:
>>>>>>>>   CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0+ #462
>>>>>>>>   Hardware name: MiTAC RAPTOR EV-883832-X3-0001/RAPTOR, BIOS 0.14 02/22/2019
>>>>>>>>   pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>>>>>> [...snip...]
>>>>>>>>   Call trace:
>>>>>>>>    acpi_ex_system_memory_space_handler+0x26c/0x2c8
>>>>>>>>    acpi_ev_address_space_dispatch+0x228/0x2c4
>>>>>>>>    acpi_ex_access_region+0x114/0x268
>>>>>>>>    acpi_ex_field_datum_io+0x128/0x1b8
>>>>>>>>    acpi_ex_extract_from_field+0x14c/0x2ac
>>>>>>>>    acpi_ex_read_data_from_field+0x190/0x1b8
>>>>>>>>    acpi_ex_resolve_node_to_value+0x1ec/0x288
>>>>>>>>    acpi_ex_resolve_to_value+0x250/0x274
>>>>>>>>    acpi_ds_evaluate_name_path+0xac/0x124
>>>>>>>>    acpi_ds_exec_end_op+0x90/0x410
>>>>>>>>    acpi_ps_parse_loop+0x4ac/0x5d8
>>>>>>>>    acpi_ps_parse_aml+0xe0/0x2c8
>>>>>>>>    acpi_ps_execute_method+0x19c/0x1ac
>>>>>>>>    acpi_ns_evaluate+0x1f8/0x26c
>>>>>>>>    acpi_ns_init_one_device+0x104/0x140
>>>>>>>>    acpi_ns_walk_namespace+0x158/0x1d0
>>>>>>>>    acpi_ns_initialize_devices+0x194/0x218
>>>>>>>>    acpi_initialize_objects+0x48/0x50
>>>>>>>>    acpi_init+0xe0/0x498
>>>>>>>>
>>>>>>>> As mentioned by Lorenzo:
>>>>>>>>    "We are forcing memory semantics mappings to PROT_NORMAL_NC, which
>>>>>>>>    eMAG does not like at all and I'd need to understand why. It looks
>>>>>>>>    like the issue happen in SystemMemory Opregion handler."
>>>>>>>>
>>>>>>>> Hence just revert it before everything is clear.
>>>>>>>>
>>>>>>>> Fixes: 437b38c51162 ("ACPI: Add memory semantics to acpi_os_map_memory()")
>>>>>>>> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
>>>>>>>> Cc: Ard Biesheuvel <ardb@kernel.org>
>>>>>>>> Cc: Hanjun Guo <guohanjun@huawei.com>
>>>>>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>>>>>> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>>>>>> Cc: Harb Abdulhamid <harb@amperecomputing.com>
>>>>>>>>
>>>>>>>> Signed-off-by: Jia He <justin.he@arm.com>
>>>>>>> Rewrote the commit log, please take the patch below and repost
>>>>>>> it as a v3.
>>>>>>>
>>>>>>> It would still be great if Ampere can help us understand why
>>>>>>> the NormalNC attributes trigger a sync abort on the opregion
>>>>>>> before merging it.
>>>>>> To be honest, I don't think you really need an explanation from Ampere
>>>>>> here.  Mapping a part of the address space that doesn't provide memory
>>>>>> semantics with NormalNC attributes is wrong and triggering a sync
>>>>>> abort in that case is way better than silently ignoring the access.
>>>>> That's understood and that's what I explained in the revert commit
>>>>> log, no question about it.
>>>>>
>>>>> I was just asking to confirm if that's what's actually happening.
>>>>>
>>>>>> Putting my OpenBSD hat on (where we have our own ACPI OSPM
>>>>>> implementation) I must say that we always interpreted SystemMemory as
>>>>>> memory mapped IO and I think that is a logical choice as SystemIO is
>>>>>> used for (non-memory mapped) IO.  And I'd say that the ACPI OSPM code
>>>>>> should make sure that it uses properly aligned access to any Field
>>>>>> object that doesn't use AnyAcc as its access type.  Even on x86!  And
>>>>>> I'd say that AML that uses AnyAcc fields for SystemMemory OpRegions on
>>>>>> arm64 is buggy.
>>>>>>
>>>>>> But maybe relaxing this when the EFI memory map indicates that the
>>>>>> address space in question does provide memory semantics does make
>>>>>> sense.  That should defenitely be documented in the ACPI standard
>>>>>> though.
>>>>> Mapping SystemMemory Opregions as "memory" does not make sense
>>>>> at all to me. Still, that's what Linux ACPICA code does (*if*
>>>>> that's what acpi_os_map_memory() is supposed to mean).
>>>>>
>>>>> https://lore.kernel.org/linux-acpi/20210916160827.GA4525@lpieralisi
>>>> It doesn't need to do that, though, if there are good enough arguments
>>>> to change the current behavior (and the argument here is that it may
>>>> be an MMIO region, so mapping it as memory doesn't really work, but it
>>>> also may be a region in memory - there is no rule in the spec by which
>>>> SystemMemory Opregions cannot be "memory" AFAICS) and if that change
>>>> doesn't introduce regressions in the installed base.
>>>>
>>>>> Where do we go from here, to be defined, we still have a bug
>>>>> to fix after the revert is applied.
>>>>>
>>>>> drivers/acpi/sysfs.c
>>>>>
>>>>> maps BERT error regions with acpi_os_map_memory().
>>>> That mechanism is basically used for exporting ACPI tables to user
>>>> space and they are known to reside in memory.  Whether or not BERT
>>>> regions should be mapped in the same way is a good question.
>>> It is not inconceivable that BERT regions actually live in memory of
>>> the BMC that is exposed over a bus that doesn't implement memory
>>> semantics is it?
>> No, it isn't, which is why I think that mapping them as RAM may not be
>> a good idea in general.
> Should I patch acpi_data_show() to map BERT error regions (well, that's
> what acpi_data_show() is used on at the moment) as MMIO and use the
> related memcpy routine to read them then :) ?

It actually would be good to clean it up so it is clear that this is 
only used for BERT.

And then there is this question: if this is not RAM (so effectively it 
is device memory), should it be exposed directly to user space?