linux-kernel - Re: [PATCH 01/22] kvm: mmu: Separate making SPTEs from set

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Date:   Wed, 30 Sep 2020 16:03:04 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...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 01/22] kvm: mmu: Separate making SPTEs from set_spte

On Tue, Sep 29, 2020 at 9:55 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Fri, Sep 25, 2020 at 02:22:41PM -0700, Ben Gardon wrote:
> > +static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
> > +                 unsigned int pte_access, int level,
> > +                 gfn_t gfn, kvm_pfn_t pfn, bool speculative,
> > +                 bool can_unsync, bool host_writable)
> > +{
> > +     u64 spte = 0;
> > +     struct kvm_mmu_page *sp;
> > +     int ret = 0;
> > +
> > +     if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
> > +             return 0;
> > +
> > +     sp = sptep_to_sp(sptep);
> > +
> > +     spte = make_spte(vcpu, pte_access, level, gfn, pfn, *sptep, speculative,
> > +                      can_unsync, host_writable, sp_ad_disabled(sp), &ret);
> > +     if (!spte)
> > +             return 0;
>
> This is an impossible condition.  Well, maybe it's theoretically possible
> if page track is active, with EPT exec-only support (shadow_present_mask is
> zero), and pfn==0.  But in that case, returning early is wrong.
>
> Rather than return the spte, what about returning 'ret', passing 'new_spte'
> as a u64 *, and dropping the bail early path?  That would also eliminate
> the minor wart of make_spte() relying on the caller to initialize 'ret'.

I agree that would make this much cleaner.

>
> > +
> > +     if (spte & PT_WRITABLE_MASK)
> > +             kvm_vcpu_mark_page_dirty(vcpu, gfn);
> > +
> >       if (mmu_spte_update(sptep, spte))
> >               ret |= SET_SPTE_NEED_REMOTE_TLB_FLUSH;
> >       return ret;
> > --
> > 2.28.0.709.gb0816b6eb0-goog
> >