Re: [PATCH 1/2] KVM: x86/mmu: Don't skip non-leaf SPTEs when zapping all SPTEs

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Ben Gardon <bgardon@google.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Don't skip non-leaf SPTEs when zapping all SPTEs
Date: Thu, 12 Aug 2021 16:43:48 +0000	[thread overview]
Message-ID: <YRVPxCv2RtyXi+XO@google.com> (raw)
In-Reply-To: <01b22936-49b0-638e-baf8-269ba93facd8@redhat.com>

On Thu, Aug 12, 2021, Paolo Bonzini wrote:
> On 12/08/21 07:07, Sean Christopherson wrote:
> > @@ -739,8 +749,16 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> >   			  gfn_t start, gfn_t end, bool can_yield, bool flush,
> >   			  bool shared)
> >   {
> > +	bool zap_all = (end == ZAP_ALL_END);
> >   	struct tdp_iter iter;
> > +	/*
> > +	 * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
> > +	 * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
> > +	 * and so KVM will never install a SPTE for such addresses.
> > +	 */
> > +	end = min(end, 1ULL << (shadow_phys_bits - PAGE_SHIFT));
> 
> Then zap_all need not have any magic value.  You can use 0/-1ull, it's
> readable enough.  ZAP_ALL_END is also unnecessary here if you do:
> 
> 	gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> 	bool zap_all = (start == 0 && end >= max_gfn_host);

Aha!  Nice.  I was both too clever and yet not clever enough.

> 	end = min(end, max_gfn_host);
> 
> And as a small commit message nit, I would say "don't leak" instead of
> "don't skip", since that's really the effect.

Hrm, yeah, I can see how "skip" doesn't raise alarm bells like it should.