linux-kernel - Re: [PATCH v5 14/19] KVM: Clean up local variable usage in __kvm_set_memory

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Date:   Thu, 6 Feb 2020 11:22:30 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Christoffer Dall <christoffer.dall@....com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>
Subject: Re: [PATCH v5 14/19] KVM: Clean up local variable usage in
 __kvm_set_memory_region()

On Thu, Feb 06, 2020 at 02:06:41PM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:52PM -0800, Sean Christopherson wrote:
> 
> [...]
> 
> > @@ -1101,52 +1099,55 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  	if (mem->guest_phys_addr + mem->memory_size < mem->guest_phys_addr)
> >  		return -EINVAL;
> >  
> > -	slot = id_to_memslot(__kvm_memslots(kvm, as_id), id);
> > -	base_gfn = mem->guest_phys_addr >> PAGE_SHIFT;
> > -	npages = mem->memory_size >> PAGE_SHIFT;
> > -
> > -	if (npages > KVM_MEM_MAX_NR_PAGES)
> > -		return -EINVAL;
> > -
> >  	/*
> >  	 * Make a full copy of the old memslot, the pointer will become stale
> >  	 * when the memslots are re-sorted by update_memslots().
> >  	 */
> > -	old = *slot;
> > +	tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
> > +	old = *tmp;
> > +	tmp = NULL;
> 
> Shall we keep this chunk to the patch where it will be used?  Other
> than that, it looks good to me.

I assume you're talking about doing this instead of using @tmp?

	old = *id_to_memslot(__kvm_memslots(kvm, as_id), id);

It's obviously possible, but I really like resulting diff for
__kvm_set_memory_region() in "KVM: Terminate memslot walks via used_slots"
when tmp is used.

@@ -1104,8 +1203,13 @@ int __kvm_set_memory_region(struct kvm *kvm,
         * when the memslots are re-sorted by update_memslots().
         */
        tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
-       old = *tmp;
-       tmp = NULL;
+       if (tmp) {
+               old = *tmp;
+               tmp = NULL;
+       } else {
+               memset(&old, 0, sizeof(old));
+               old.id = id;
+       }