Re: [PATCH v4 02/12] x86/xen: simplify flush_lazy_mmu()

[Ryan Roberts <ryan.roberts@xxxxxxx>]

On 10/11/2025 10:36, Kevin Brodsky wrote:
> On 07/11/2025 12:31, Ryan Roberts wrote:
>> On 29/10/2025 10:08, Kevin Brodsky wrote:
>>> arch_flush_lazy_mmu_mode() is called when outstanding batched
>>> pgtable operations must be completed immediately. There should
>>> however be no need to leave and re-enter lazy MMU completely. The
>>> only part of that sequence that we really need is xen_mc_flush();
>>> call it directly.
>>>
>>> Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
>> This looks functionally equivalent to me, so:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>
>> But I don't think this tidy up is strictly necessary for your series to work?
>> (perhaps I'll change my mind on that as I go through it).
> 
> I initially thought it might be, but in the end I think you're right -
> it should still work fine without this patch.
> 
> Still, I'd rather avoid unnecessary calls to arch_enter() and
> arch_leave() as it makes it harder to reason about what is called where.
> Namely, keeping them here means that a nested call to
> lazy_mmu_mode_disable() would cause arch_leave() then arch_enter() to be
> called - rather unexpected.
> 
> The only calls to arch_enter() and arch_leave() that are left after this
> series are the ones in <linux/pgtable.h> and the Xen context-switching
> logic (the one case where calling arch hooks directly is justified, see
> discussion on v3 [1]).

OK yeah, sounds reasonable.

> 
> - Kevin
> 
> [1]
> https://lore.kernel.org/all/390e41ae-4b66-40c1-935f-7a1794ba0b71@xxxxxxx/