linux-kernel - Re: [PATCH] KVM: const-ify all relevant uses of struct kvm_memory

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Message-ID: <YQReyaxp/rwypHbR@t490s>
Date:   Fri, 30 Jul 2021 16:19:21 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Hamza Mahfooz <someguy@...ective-light.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: const-ify all relevant uses of struct
 kvm_memory_slot

Hi, Hamza,

On Mon, Jul 12, 2021 at 10:33:38PM -0400, Hamza Mahfooz wrote:
> @@ -1467,16 +1467,20 @@ rmap_walk_init_level(struct slot_rmap_walk_iterator *iterator, int level)
>  
>  static void
>  slot_rmap_walk_init(struct slot_rmap_walk_iterator *iterator,
> -		    struct kvm_memory_slot *slot, int start_level,
> +		    const struct kvm_memory_slot *slot, int start_level,
>  		    int end_level, gfn_t start_gfn, gfn_t end_gfn)
>  {
> -	iterator->slot = slot;
> -	iterator->start_level = start_level;
> -	iterator->end_level = end_level;
> -	iterator->start_gfn = start_gfn;
> -	iterator->end_gfn = end_gfn;
> +	struct slot_rmap_walk_iterator iter = {
> +		.slot = slot,
> +		.start_gfn = start_gfn,
> +		.end_gfn = end_gfn,
> +		.start_level = start_level,
> +		.end_level = end_level,
> +	};
> +
> +	rmap_walk_init_level(&iter, iterator->start_level);

Here it should be s/iterator->//.

>  
> -	rmap_walk_init_level(iterator, iterator->start_level);
> +	memcpy(iterator, &iter, sizeof(struct slot_rmap_walk_iterator));
>  }

This patch breaks kvm/queue with above issue.  Constify of kvm_memory_slot
pointer should have nothing to do with this so at least it should need a
separate patch.  At the meantime I also don't understand why memcpy() here,
which seems to be even slower..

-- 
Peter Xu