Re: [PATCH 4/8] xen/common: Split tlb-clock.h out of mm.h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/03/2025 7:43 pm, Nicola Vetrini wrote:
> On 2025-03-13 14:35, Andrew Cooper wrote:
>> On 13/03/2025 12:59 pm, Jan Beulich wrote:
>>> On 12.03.2025 18:45, Andrew Cooper wrote:
>>>> xen/mm.h includes asm/tlbflush.h almost at the end, which creates a
>>>> horrible
>>>> tangle.  This is in order to provide two common files with an
>>>> abstraction over
>>>> the x86-specific TLB clock logic.
>>>>
>>>> First, introduce CONFIG_HAS_TLB_CLOCK, selected by x86 only.  Next,
>>>> introduce
>>>> xen/tlb-clock.h, providing empty stubs, and include this into
>>>> memory.c and
>>>> page_alloc.c
>>>>
>>>> No functional change.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>>>> CC: Michal Orzel <michal.orzel@xxxxxxx>
>>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>>> CC: Julien Grall <julien@xxxxxxx>
>>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>>> CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
>>>> CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>>> CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
>>>> CC: Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>
>>>>
>>>> There is still a mess here with the common vs x86 split, but it's
>>>> better
>>>> contained than before.
>>>> ---
>>>>  xen/arch/x86/Kconfig        |  1 +
>>>>  xen/common/Kconfig          |  3 +++
>>>>  xen/common/memory.c         |  1 +
>>>>  xen/common/page_alloc.c     |  1 +
>>>>  xen/include/xen/mm.h        | 27 --------------------
>>>>  xen/include/xen/tlb-clock.h | 49
>>>> +++++++++++++++++++++++++++++++++++++
>>>>  6 files changed, 55 insertions(+), 27 deletions(-)
>>>>  create mode 100644 xen/include/xen/tlb-clock.h
>>>>
>
>
>>> However, see below.
>>>
>>>> +        arch_flush_tlb_mask(&mask);
>>>> +    }
>>>> +}
>>>> +
>>>> +#else /* !CONFIG_HAS_TLB_CLOCK */
>>>> +
>>>> +struct page_info;
>>>> +static inline void accumulate_tlbflush(
>>>> +    bool *need_tlbflush, const struct page_info *page,
>>>> +    uint32_t *tlbflush_timestamp) {}
>>>> +static inline void filtered_flush_tlb_mask(uint32_t
>>>> tlbflush_timestamp) {}
>>> Is doing nothing here correct?
>>
>> Yeah, it's not, but this only occurred to me after sending the series.
>>
>> Interestingly, CI is green across the board for ARM, which suggests to
>> me that this logic isn't getting a workout.
>>
>>>  mark_page_free() can set a page's
>>> ->u.free.need_tlbflush. And with that flag set the full
>>>
>>> static inline void accumulate_tlbflush(
>>>     bool *need_tlbflush, const struct page_info *page,
>>>     uint32_t *tlbflush_timestamp)
>>> {
>>>     if ( page->u.free.need_tlbflush &&
>>>          page->tlbflush_timestamp <= tlbflush_current_time() &&
>>>          (!*need_tlbflush ||
>>>           page->tlbflush_timestamp > *tlbflush_timestamp) )
>>>     {
>>>         *need_tlbflush = true;
>>>         *tlbflush_timestamp = page->tlbflush_timestamp;
>>>     }
>>> }
>>>
>>> reduces to (considering that tlbflush_current_time() resolves to
>>> constant 0,
>>> which also implies every page's ->tlbflush_timestamp is only ever 0)
>>>
>>> static inline void accumulate_tlbflush(
>>>     bool *need_tlbflush, const struct page_info *page,
>>>     uint32_t *tlbflush_timestamp)
>>> {
>>>     if ( !*need_tlbflush )
>>>         *need_tlbflush = true;
>>> }
>>>
>>> which means a not-stubbed-out filtered_flush_tlb_mask(), with
>>> tlbflush_filter()
>>> doing nothing, would actually invoke arch_flush_tlb_mask() (with all
>>> online CPUs
>>> set in the mask) when called. And arch_flush_tlb_mask() isn't a
>>> no-op on Arm.
>>
>> Yes.  Sadly, fixing this (without Eclair complaining in the middle of
>> the series) isn't as easy as I'd hoped.
>>
>
> Hi Andrew,
>
> I didn't quite follow the whole thread (been busy the last couple of
> days), but could you explain briefly what's the issue here? Just a
> link to a failing pipeline should be fine as well.

There isn't one.

But to untangle this the easy way, I'd need to have a duplicate
declaration for arch_flush_tlb_mask() for a patch of two.

Which I know Eclair would complain about, and therefore I need to find a
different way to untangle it.

~Andrew