linux-kernel - Re: [PATCH 6/6] KVM: x86/mmu: Lazily allocate memslot rmaps

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Message-ID: <CANgfPd_o+UA4ry7Kpw3WbcPZYm32r+1o=hQmZdazsrZvO4aynA@mail.gmail.com>
Date:   Wed, 28 Apr 2021 09:45:56 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 6/6] KVM: x86/mmu: Lazily allocate memslot rmaps

On Wed, Apr 28, 2021 at 3:04 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 28/04/21 00:36, Ben Gardon wrote:
> > -static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> > +static int kvm_alloc_memslot_metadata(struct kvm *kvm,
> > +                                   struct kvm_memory_slot *slot,
> >                                     unsigned long npages)
> >   {
> >       int i;
> > @@ -10892,7 +10950,7 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> >        */
> >       memset(&slot->arch, 0, sizeof(slot->arch));
> >
> > -     r = alloc_memslot_rmap(slot, npages);
> > +     r = alloc_memslot_rmap(kvm, slot, npages);
> >       if (r)
> >               return r;
> >
>
> I wonder why you need alloc_memslot_rmap at all here in
> kvm_alloc_memslot_metadata, or alternatively why you need to do it in
> kvm_arch_assign_memslots.  It seems like only one of those would be
> necessary.

Oh, that's a good point. We need it in kvm_arch_assign_memslots
because of the race I identified in the thread for patch 5 of this
series, but we could remove it from kvm_alloc_memslot_metadata with
this patch.

Of course, it would be much nicer if we could just keep it in
kvm_alloc_memslot_metadata and find some other way to stop memslots
without rmaps from being inappropriately installed, if anyone has a
simpler way to accomplish that.

>
> Paolo
>