linux-kernel - Re: [PATCH v5 17/19] KVM: Terminate memslot walks via used

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Message-ID: <20200207222438.GH720553@xz-x1>
Date:   Fri, 7 Feb 2020 17:24:38 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 17/19] KVM: Terminate memslot walks via used_slots

On Fri, Feb 07, 2020 at 02:03:25PM -0800, Sean Christopherson wrote:
> On Fri, Feb 07, 2020 at 04:46:23PM -0500, Peter Xu wrote:
> > On Fri, Feb 07, 2020 at 01:10:16PM -0800, Sean Christopherson wrote:
> > > On Fri, Feb 07, 2020 at 03:39:09PM -0500, Peter Xu wrote:
> > > > On Fri, Feb 07, 2020 at 10:33:25AM -0800, Sean Christopherson wrote:
> > > > > On Thu, Feb 06, 2020 at 04:09:44PM -0500, Peter Xu wrote:
> > > > > > On Tue, Jan 21, 2020 at 02:31:55PM -0800, Sean Christopherson wrote:
> > > > > > > @@ -9652,13 +9652,13 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
> > > > > > >  		if (IS_ERR((void *)hva))
> > > > > > >  			return PTR_ERR((void *)hva);
> > > > > > >  	} else {
> > > > > > > -		if (!slot->npages)
> > > > > > > +		if (!slot || !slot->npages)
> > > > > > >  			return 0;
> > > > > > >  
> > > > > > > -		hva = 0;
> > > > > > > +		hva = slot->userspace_addr;
> > > > > > 
> > > > > > Is this intended?
> > > > > 
> > > > > Yes.  It's possible to allow VA=0 for userspace mappings.  It's extremely
> > > > > uncommon, but possible.  Therefore "hva == 0" shouldn't be used to
> > > > > indicate an invalid slot.
> > > > 
> > > > Note that this is the deletion path in __x86_set_memory_region() not
> > > > allocation.  IIUC userspace_addr won't even be used in follow up code
> > > > path so it shouldn't really matter.  Or am I misunderstood somewhere?
> > > 
> > > No, but that's precisely why I don't want to zero out @hva, as doing so
> > > implies that '0' indicates an invalid hva, which is wrong.
> > > 
> > > What if I change this to 
> > > 
> > > 			hva = 0xdeadull << 48;
> > > 
> > > and add a blurb in the changelog about stuff hva with a non-canonical value
> > > to indicate it's being destroyed.
> > 
> > IMO it's fairly common to have the case where "when A is XXX then
> > parameters B is invalid" happens in C.
> 
> I'm not arguing that's not the case.  My point is that there's nothing
> special about '0', so why use it?  E.g. "hva = 1" would also be ok from a
> functional perspective, but more obviously "wrong".

I think the answer is as simple as the original author thought 0 was
better than an arbitrary number on the stack, which I agree. :-)

-- 
Peter Xu