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

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 01/11/2025 12:14, David Hildenbrand wrote:
> On 29.10.25 11:08, 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@arm.com>
>> ---
>>   arch/x86/xen/mmu_pv.c | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
>> index 2a4a8deaf612..7a35c3393df4 100644
>> --- a/arch/x86/xen/mmu_pv.c
>> +++ b/arch/x86/xen/mmu_pv.c
>> @@ -2139,10 +2139,8 @@ static void xen_flush_lazy_mmu(void)
>>   {
>>       preempt_disable();
>>   -    if (xen_get_lazy_mode() == XEN_LAZY_MMU) {
>> -        arch_leave_lazy_mmu_mode();
>> -        arch_enter_lazy_mmu_mode();
>> -    }
>> +    if (xen_get_lazy_mode() == XEN_LAZY_MMU)
>> +        xen_mc_flush();
>>         preempt_enable();
>>   }
>
> Looks like that was moved to XEN code in
>
> commit a4a7644c15096f57f92252dd6e1046bf269c87d8
> Author: Juergen Gross <jgross@suse.com>
> Date:   Wed Sep 13 13:38:27 2023 +0200
>
>     x86/xen: move paravirt lazy code
>
>
> And essentially the previous implementation lived in
> arch/x86/kernel/paravirt.c:paravirt_flush_lazy_mmu(void) in an
> implementation-agnostic way:
>
> void paravirt_flush_lazy_mmu(void)
> {
>        preempt_disable();
>
>        if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
>                arch_leave_lazy_mmu_mode();
>                arch_enter_lazy_mmu_mode();
>        }
>
>        preempt_enable();
> }

Indeed, I saw that too. Calling the generic leave/enter functions made
some sense at that point, but now that the implementation is
Xen-specific we can directly call xen_mc_flush().

>
> So indeed, I assume just doing the flush here is sufficient.
>
> Reviewed-by: David Hildenbrand <david@redhat.com> 

Thanks for the review!

- Kevin