linux-kernel - Re: [PATCH] KVM: Wrap kvm_{gfn,hva}

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Message-ID: <ZMfxYR41K71UV/84@linux.dev>
Date:   Mon, 31 Jul 2023 17:37:37 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH] KVM: Wrap kvm_{gfn,hva}_range.pte in a per-action union

On Fri, Jul 28, 2023 at 05:41:44PM -0700, Sean Christopherson wrote:
> Wrap kvm_{gfn,hva}_range.pte in a union so that future notifier events can
> pass event specific information up and down the stack without needing to
> constantly expand and churn the APIs.  Lockless aging of SPTEs will pass
> around a bitmap, and support for memory attributes will pass around the
> new attributes for the range.
> 
> Add a "KVM_NO_ARG" placeholder to simplify handling events without an
> argument (creating a dummy union variable is midly annoying).
> 
> Opportunstically drop explicit zero-initialization of the "pte" field, as
> omitting the field (now a union) has the same effect.
> 
> Cc: Yu Zhao <yuzhao@...gle.com>
> Link: https://lore.kernel.org/all/CAOUHufagkd2Jk3_HrVoFFptRXM=hX2CV8f+M-dka-hJU4bP8kw@mail.gmail.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Looks good, and I don't think it'll conflict with anything on the arm64
side.

Reviewed-by: Oliver Upton <oliver.upton@...ux.dev>

> ---
> 
> If this looks good, my thought is to squeeze it into 6.6 so that the MGLRU
> and guest_memfd() series can build on it.  Or those series could just
> include it?

Eh, I'm not a huge fan of having two series independently reposting a
common base. It can be a bit annoying when the two authors have slightly
different interpretations on how to improve it...

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index dfbaafbe3a00..f84ef9399aee 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -526,7 +526,7 @@ typedef void (*on_unlock_fn_t)(struct kvm *kvm);
>  struct kvm_hva_range {
>  	unsigned long start;
>  	unsigned long end;
> -	pte_t pte;
> +	union kvm_mmu_notifier_arg arg;
>  	hva_handler_t handler;
>  	on_lock_fn_t on_lock;
>  	on_unlock_fn_t on_unlock;
> @@ -547,6 +547,8 @@ static void kvm_null_fn(void)
>  }
>  #define IS_KVM_NULL_FN(fn) ((fn) == (void *)kvm_null_fn)
>  
> +static const union kvm_mmu_notifier_arg KVM_NO_ARG;
> +

I'm guessing you were trying to keep this short, but it might be nice to
use MMU_NOTIFIER_ (or similar) as the prefix to make the scope
immediately obvious.

-- 
Thanks,
Oliver