Re: [PATCH 4/7] x86/xen: support nested lazy_mmu sections (again)

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 05/09/2025 17:48, Alexander Gordeev wrote:
> On Thu, Sep 04, 2025 at 01:57:33PM +0100, Kevin Brodsky wrote:
> ...
>> -static void xen_enter_lazy_mmu(void)
>> +static lazy_mmu_state_t xen_enter_lazy_mmu(void)
>>  {
>> +    if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
>> +            return LAZY_MMU_NESTED;
>> +
>>      enter_lazy(XEN_LAZY_MMU);
>> +    return LAZY_MMU_DEFAULT;
>>  }
>>  
>>  static void xen_flush_lazy_mmu(void)
>> @@ -2167,11 +2171,12 @@ static void __init xen_post_allocator_init(void)
>>      pv_ops.mmu.write_cr3 = &xen_write_cr3;
>>  }
>>  
>> -static void xen_leave_lazy_mmu(void)
>> +static void xen_leave_lazy_mmu(lazy_mmu_state_t state)
>>  {
>>      preempt_disable();
>>      xen_mc_flush();
>> -    leave_lazy(XEN_LAZY_MMU);
>> +    if (state != LAZY_MMU_NESTED)
>> +            leave_lazy(XEN_LAZY_MMU);
> Based on xen_enter_lazy_mmu(), whether this condition needs to be
> executed with the preemption disabled?

AFAIU xen_mc_flush() needs preemption to be disabled. I don't think
{enter,leave}_lazy() do, but this patch doesn't introduce any change
from that perspective. I suppose it doesn't hurt that
xen_leave_lazy_mmu() calls leave_lazy() with preemption disabled.

> Or may be this_cpu_read(xen_lazy_mode) + enter_lazy(XEN_LAZY_MMU)
> should be executed with the preemption disabled?

Adding another this_cpu_read(xen_lazy_mode) in xen_enter_lazy_mmu()
shouldn't change the situation, i.e. preemption should still be safe. If
preemption occurs in the middle of that function,
xen_{start,end}_context_switch() will do the right thing to save/restore
xen_lazy_mode.

- Kevin