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Tsirkin" , Jason Wang , Christophe de Dinechin , Yan Zhao , Alex Williamson , Paolo Bonzini , Vitaly Kuznetsov , "Dr . David Alan Gilbert" Subject: Re: [PATCH v8 03/14] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR] Message-ID: <20200424152151.GB41816@xz-x1> References: <20200331190000.659614-1-peterx@redhat.com> <20200331190000.659614-4-peterx@redhat.com> <20200423203944.GS17824@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200423203944.GS17824@linux.intel.com> Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Thu, Apr 23, 2020 at 01:39:44PM -0700, Sean Christopherson wrote: > On Tue, Mar 31, 2020 at 02:59:49PM -0400, Peter Xu wrote: > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > index 1b6d9ac9533c..faa702c4d37b 100644 > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -9791,7 +9791,32 @@ void kvm_arch_sync_events(struct kvm *kvm) > > kvm_free_pit(kvm); > > } > > > > -int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size) > > +#define ERR_PTR_USR(e) ((void __user *)ERR_PTR(e)) > > Heh, my first thought when reading the below code was "cool, I didn't know > there was ERR_PTR_USR!". This probably should be in include/linux/err.h, > or maybe a new arch specific implementation if it's not universally safe. Yeah, I just wanted to avoid introducing things in common headers before I'm sure it'll be used in the rest of the world.. We can always replace them with a global definition when it comes. > > An alternative, which looks enticing given that proper user variants will > be a bit of an explosion, would be to do: > > static void *____x86_set_memory_region(...) > { > > } > > void __user *__x86_set_memory_region(...) > { > return (void __user *)____x86_set_memory_region(...); > } > > A second alternative would be to return an "unsigned long", i.e. force the > one function that actually accesses the hva to do the cast. I think I like > this option the best as it would minimize the churn in > __x86_set_memory_region(). Callers can use IS_ERR_VALUE() to detect failure. If you won't mind, I would prefer a 2nd opinion (maybe Paolo?) so we can consolidate the idea before I change them... (I would for sure still prefer the current approach for simplicity since after all I don't have strong opionion..) > > > +/** > > + * __x86_set_memory_region: Setup KVM internal memory slot > > + * > > + * @kvm: the kvm pointer to the VM. > > + * @id: the slot ID to setup. > > + * @gpa: the GPA to install the slot (unused when @size == 0). > > + * @size: the size of the slot. Set to zero to uninstall a slot. > > + * > > + * This function helps to setup a KVM internal memory slot. Specify > > + * @size > 0 to install a new slot, while @size == 0 to uninstall a > > + * slot. The return code can be one of the following: > > + * > > + * HVA: on success (uninstall will return a bogus HVA) > > I think it's important to call out that it returns '0' on uninstall, e.g. > otherwise it's not clear how a caller can detect failure. It will "return (0xdeadull << 48)" as you proposed in abbed4fa94f6? :-) Frankly speaking I always preferred zero but that's just not true any more after above change. This also reminded me that maybe we should also return the same thing at [1] below. > > > + * -errno: on error > > + * > > + * The caller should always use IS_ERR() to check the return value > > + * before use. Note, the KVM internal memory slots are guaranteed to > > + * remain valid and unchanged until the VM is destroyed, i.e., the > > + * GPA->HVA translation will not change. However, the HVA is a user > > + * address, i.e. its accessibility is not guaranteed, and must be > > + * accessed via __copy_{to,from}_user(). > > + */ > > +void __user * __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, > > + u32 size) > > { > > int i, r; > > unsigned long hva, uninitialized_var(old_npages); > > @@ -9800,12 +9825,12 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size) > > > > /* Called with kvm->slots_lock held. */ > > if (WARN_ON(id >= KVM_MEM_SLOTS_NUM)) > > - return -EINVAL; > > + return ERR_PTR_USR(-EINVAL); > > > > slot = id_to_memslot(slots, id); > > if (size) { > > if (slot && slot->npages) > > - return -EEXIST; > > + return ERR_PTR_USR(-EEXIST); > > > > /* > > * MAP_SHARED to prevent internal slot pages from being moved > > @@ -9814,10 +9839,10 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size) > > hva = vm_mmap(NULL, 0, size, PROT_READ | PROT_WRITE, > > MAP_SHARED | MAP_ANONYMOUS, 0); > > if (IS_ERR((void *)hva)) > > IS_ERR_VALUE() can be used to avoid the double cast. Agreed. But it's a context cleanup, so I normally will keep it as is (or use a standalone patch). > > > - return PTR_ERR((void *)hva); > > + return (void __user *)hva; > > If we still want to go down the route of ERR_PTR_USR, then an ERR_CAST_USR > seems in order. Sure. But I'll still keep it kvm-only if you won't mind... > > > } else { > > if (!slot || !slot->npages) > > - return 0; > > + return ERR_PTR_USR(0); [1] > > "return ERR_PTR_USR(NULL)" or "return NULL" would be more intuitive. Moot > point if the return is changed to "unsigned long". ERR_PTR_USR() takes a "long". I can use ERR_CAST_USR(NULL) if you prefer me to explicitly use NULL. Thanks, -- Peter Xu