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

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Date:   Tue, 29 Sep 2020 21:55:08 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, 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 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'.

> +
> +	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
>