Re: [PATCH v3 1/2] x86/mm: rename FLUSH_FORCE_IPI to FLUSH_NO_ASSIST

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.05.2022 16:33, Andrew Cooper wrote:
> On 25/05/2022 12:14, Jan Beulich wrote:
>> On 25.05.2022 12:52, Andrew Cooper wrote:
>>> On 25/05/2022 09:13, Roger Pau Monne wrote:
>>>> Rename the flag to better note that it's not actually forcing any IPIs
>>>> to be issued if none is required, but merely avoiding the usage of TLB
>>>> flush assistance (which itself can avoid the sending of IPIs to remote
>>>> processors).
>>>>
>>>> No functional change expected.
>>>>
>>>> Requested-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>> ---
>>>> Changes since v2:
>>>>  - New in this version.
>>> :(  This needs reverting.
>>>
>>> It is specific to IPIs, because of our current choice of algorithm for
>>> freeing pagetables.
>>>
>>> "no assist" excludes ipi-helper hypercalls which invoke
>>> INVALIDATE_TLB_VECTOR.  Such hypercalls do exist and specifically would
>>> be improvement that we ought to use.
>>>
>>> Furthermore, we do want to work around the limitation that created
>>> FLUSH_FORCE_IPI, because we absolutely do want to be able to use
>>> hypercalls/remote TLB flushing capabilities when available.
>>>
>>> I accept that FORCE_IPI perhaps isn't a perfect name, but it's a whole
>>> lot less bad than NO_ASSIST.
>> But FORCE_IPI has caused actual confusion while reviewing patch 2.
>> If NO_ASSIST doesn't suit you and FORCE_IPI is also wrong, can you
>> suggest a better name fitting both aspects?
> 
> I don't actually agree that FORCE_IPI is unclear to begin with.

You did see the earlier communication with Roger, didn't you? To
me the name pretty clearly suggests "always IPI" (hence "force"),
i.e. ...

> The safety property required is "if you need to contact a remote CPU, it
> must be by IPI to interlock with Xen's #PF handler".

... not in any way limited to remote CPUs. Yet patch 2 is about
cases where things are safe because no IPI will be needed (not
even a self-IPI).

> FORCE_IPI is very close to meaning this.  If anything else is unclear,
> it can be clarified in the adjacent comment.

I'm afraid I don't think a comment is what would help here.

Jan