Re: [for-4.20] Re: [PATCH v12 12/12] xen/arm: add cache coloring support for Xen image

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.01.2025 17:51, Michal Orzel wrote:
> 
> 
> On 07/01/2025 17:42, Julien Grall wrote:
>>
>>
>> Hi,
>>
>> On 16/12/2024 14:36, Jan Beulich wrote:
>>> On 16.12.2024 15:28, Carlo Nonato wrote:
>>>> On Mon, Dec 16, 2024 at 2:56 PM Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>>>> On 13/12/2024 17:28, Carlo Nonato wrote:
>>>>>> --- a/xen/arch/arm/arm64/mmu/mm.c
>>>>>> +++ b/xen/arch/arm/arm64/mmu/mm.c
>>>>>> @@ -1,6 +1,7 @@
>>>>>>   /* SPDX-License-Identifier: GPL-2.0-only */
>>>>>>
>>>>>>   #include <xen/init.h>
>>>>>> +#include <xen/llc-coloring.h>
>>>>>>   #include <xen/mm.h>
>>>>>>   #include <xen/pfn.h>
>>>>>>
>>>>>> @@ -138,8 +139,36 @@ void update_boot_mapping(bool enable)
>>>>>>   }
>>>>>>
>>>>>>   extern void switch_ttbr_id(uint64_t ttbr);
>>>>>> +extern void relocate_xen(uint64_t ttbr, void *src, void *dst, size_t 
>>>>>> len);
>>>>>>
>>>>>>   typedef void (switch_ttbr_fn)(uint64_t ttbr);
>>>>>> +typedef void (relocate_xen_fn)(uint64_t ttbr, void *src, void *dst, 
>>>>>> size_t len);
>>>>>> +
>>>>>> +void __init relocate_and_switch_ttbr(uint64_t ttbr) {
>>>>> CODING_STYLE: { needs to be on its own line
>>>>>
>>>>> Also, this function is only executed in case of LLC coloring, so 
>>>>> shouldn't it
>>>>> be #ifdef protected.
>>>>
>>>> Here and in other places (patch #8) I'm relying on DCE to remove functions
>>>> that are not called. This was a suggestion from Jan in that patch. Can we
>>>> adopt the same here?
>>>
>>> Yet how would the compiler spot that the function is unused? That would only
>>> work with LTO / WPO. DCE (as I did suggest elsewhere) requires the functions
>>> in question to be static (allowing the compiler to see enough of the whole
>>> picture).
>>
>> Sorry for the late answer. I was away with limited e-mail access. While
>> looking what was committing recently, I noticed that a dummy function
>> was introduced:
>>
>> void __init relocate_and_switch_ttbr(uint64_t ttbr) {}
>>
>> If a function is not supposed to be called, then it should contain a
>> BUG_ON() to catch any misusage. Otherwise, this is a recipe for
>> disaster. In this case, it would not be trivial to notice the TTBR was
>> not switched...
>>
>> That said I would have actually considered to remove the empty stub. I
>> am a bit surprised that DCE wouldn't work in this case because the call
>> is protected with "if ( llc_coloring_enabled )". When cache coloring is
>> not enabled, this would turn to an "if ( false )" and therefore all the
>> code should be removed. What did I miss?
>>
>> Note that this is what we already rely on for arm32 because there is no
>> stub... So if this is problem then we definitely need to fix it on arm32
>> as well...
>>
>> IOW, we either introduce a stub (including the BUG_ON) for both arm32
>> and arm64 in the header or we remove the stub completely.
>>
>> Marco, Michal, can you have a look? Ideally, this should be fixed for 4.20.
> I did a test with GCC 13.2 and I can compile it fine with stub removed. That 
> said,
> I'm not a compiler expert and I'm not sure if this behavior stays the same 
> with different
> compiler options/optimizations. So it's more like a question to Jan. I'm 
> happy either way.

We use the same (if(...) func();) in various places, relying on said DCEing
of the call when the condition is compile-time-false. I see no reason why
it couldn't be used here as well.

Jan