Re: [PATCH kernel v2] KVM: PPC: Make KVM_CAP_IRQFD_RESAMPLE support platform dependent

On 21/09/2022 02:08, Marc Zyngier wr
ote:
> On Tue, 20 Sep 2022 13:51:43 +0100,
> Alexey Kardashevskiy <[email protected]> wrote:
>>
>> When introduced, IRQFD resampling worked on POWER8 with XICS. However
>> KVM on POWER9 has never implemented it - the compatibility mode code
>> ("XICS-on-XIVE") misses the kvm_notify_acked_irq() call and the native
>> XIVE mode does not handle INTx in KVM at all.
>>
>> This moved the capability support advertising to platforms and stops
>> advertising it on XIVE, i.e. POWER9 and later.
>>
>> Signed-off-by: Alexey Kardashevskiy <[email protected]>
>> Acked-by: Nicholas Piggin <[email protected]>
>> [For KVM RISC-V]
>> Acked-by: Anup Patel <[email protected]>
>> ---
>> Changes:
>> v2:
>> * removed ifdef for ARM64.
> 
> The same argument applies to both x86 and s390, which do select
> HAVE_KVM_IRQFD from the KVM config option. Only power allows this
> option to be selected depending on the underlying interrupt controller
> emulation.
> 
> As for riscv and mips, they don't select HAVE_KVM_IRQFD, and this
> isn't a user-selectable option. So why do they get patched at all?

Before the patch, the capability was advertised on those, with your 
proposal it will stop. Which is a change in behavior which some may say 
requires a separate patch (like, one per platform). I am definitely 
overthinking it though and you are right.

> My conclusion is that:
> 
> - only power needs the #ifdefery in the arch-specific code
> - arm64, s390 and x86 can use KVM_CAP_IRQFD_RESAMPLE without a #ifdef
> - mips and riscv should be left alone

Fair enough, thanks for the review! I'll post v3 shortly.

> 
> Thanks,
> 
> 	M.
> 

-- 
Alexey