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Fri, 07 Feb 2020 14:24:41 -0800 (PST) Date: Fri, 7 Feb 2020 17:24:38 -0500 From: Peter Xu To: Sean Christopherson Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Christoffer Dall , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Subject: Re: [PATCH v5 17/19] KVM: Terminate memslot walks via used_slots Message-ID: <20200207222438.GH720553@xz-x1> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-18-sean.j.christopherson@intel.com> <20200206210944.GD700495@xz-x1> <20200207183325.GI2401@linux.intel.com> <20200207203909.GE720553@xz-x1> <20200207211016.GN2401@linux.intel.com> <20200207214623.GF720553@xz-x1> <20200207220325.GO2401@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200207220325.GO2401@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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