From: Peter Xu <peterx@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Kevin Tian <kevin.tian@intel.com>,
"Michael S . Tsirkin" <mst@redhat.com>,
Jason Wang <jasowang@redhat.com>,
Christophe de Dinechin <dinechin@redhat.com>,
Yan Zhao <yan.y.zhao@intel.com>,
Alex Williamson <alex.williamson@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH v8 03/14] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]
Date: Fri, 24 Apr 2020 11:21:51 -0400 [thread overview]
Message-ID: <20200424152151.GB41816@xz-x1> (raw)
In-Reply-To: <20200423203944.GS17824@linux.intel.com>
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(...)
> {
> <actual function>
> }
>
> 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
next prev parent reply other threads:[~2020-04-24 15:22 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-31 18:59 [PATCH v8 00/14] KVM: Dirty ring interface Peter Xu
2020-03-31 18:59 ` [PATCH v8 01/14] KVM: X86: Change parameter for fast_page_fault tracepoint Peter Xu
2020-03-31 18:59 ` [PATCH v8 02/14] KVM: Cache as_id in kvm_memory_slot Peter Xu
2020-03-31 18:59 ` [PATCH v8 03/14] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR] Peter Xu
2020-04-23 20:39 ` Sean Christopherson
2020-04-24 15:21 ` Peter Xu [this message]
2020-04-27 18:10 ` Sean Christopherson
2020-04-28 20:22 ` Peter Xu
2020-03-31 18:59 ` [PATCH v8 04/14] KVM: Pass in kvm pointer into mark_page_dirty_in_slot() Peter Xu
2020-03-31 18:59 ` [PATCH v8 05/14] KVM: X86: Implement ring-based dirty memory tracking Peter Xu
2020-03-31 18:59 ` [PATCH v8 06/14] KVM: Make dirty ring exclusive to dirty bitmap log Peter Xu
2020-03-31 18:59 ` [PATCH v8 07/14] KVM: Don't allocate dirty bitmap if dirty ring is enabled Peter Xu
2020-03-31 18:59 ` [PATCH v8 08/14] KVM: selftests: Always clear dirty bitmap after iteration Peter Xu
2020-04-01 7:04 ` Andrew Jones
2020-03-31 18:59 ` [PATCH v8 09/14] KVM: selftests: Sync uapi/linux/kvm.h to tools/ Peter Xu
2020-03-31 18:59 ` [PATCH v8 10/14] KVM: selftests: Use a single binary for dirty/clear log test Peter Xu
2020-03-31 18:59 ` [PATCH v8 11/14] KVM: selftests: Introduce after_vcpu_run hook for dirty " Peter Xu
2020-04-01 7:03 ` Andrew Jones
2020-04-01 23:24 ` Peter Xu
2020-03-31 18:59 ` [PATCH v8 12/14] KVM: selftests: Add dirty ring buffer test Peter Xu
2020-03-31 18:59 ` [PATCH v8 13/14] KVM: selftests: Let dirty_log_test async for dirty ring test Peter Xu
2020-04-01 7:48 ` Andrew Jones
2020-03-31 19:00 ` [PATCH v8 14/14] KVM: selftests: Add "-c" parameter to dirty log test Peter Xu
2020-04-22 18:51 ` [PATCH v8 00/14] KVM: Dirty ring interface Peter Xu
2020-04-23 6:28 ` Tian, Kevin
2020-04-23 15:22 ` Peter Xu
2020-04-24 6:01 ` Tian, Kevin
2020-04-24 14:19 ` Peter Xu
2020-04-26 10:29 ` Tian, Kevin
2020-04-27 14:27 ` Peter Xu
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