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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Peter Xu <peterx@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Christophe de Dinechin <dinechin@redhat.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Yan Zhao <yan.y.zhao@intel.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Kevin Kevin <kevin.tian@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]
Date: Fri, 31 Jan 2020 13:29:28 -0800
Message-ID: <20200131212928.GH18946@linux.intel.com> (raw)
In-Reply-To: <20200131205550.GB7063@xz-x1>

On Fri, Jan 31, 2020 at 03:55:50PM -0500, Peter Xu wrote:
> On Fri, Jan 31, 2020 at 12:36:22PM -0800, Sean Christopherson wrote:
> > On Fri, Jan 31, 2020 at 03:28:24PM -0500, Peter Xu wrote:
> > > On Fri, Jan 31, 2020 at 11:33:01AM -0800, Sean Christopherson wrote:
> > > > For the same reason we don't take mmap_sem, it gains us nothing, i.e. KVM
> > > > still has to use copy_{to,from}_user().
> > > > 
> > > > In the proposed __x86_set_memory_region() refactor, vmx_set_tss_addr()
> > > > would be provided the hva of the memory region.  Since slots_lock and SRCU
> > > > only protect gfn->hva, why would KVM take slots_lock since it already has
> > > > the hva?
> > > 
> > > OK so you're suggesting to unlock the lock earlier to not cover
> > > init_rmode_tss() rather than dropping the whole lock...  Yes it looks
> > > good to me.  I think that's the major confusion I got.
> > 
> > Ya.  And I missed where the -EEXIST was coming from.  I think we're on the
> > same page.
> 
> Good to know.  Btw, for me I would still prefer to keep the lock be
> after the __copy_to_user()s because "HVA is valid without lock" is
> only true for these private memslots.

No.  From KVM's perspective, the HVA is *never* valid.  Even if you rewrote
this statement to say "the gfn->hva translation is valid without lock" it
would still be incorrect. 

KVM is *always* using HVAs without holding lock, e.g. every time it enters
the guest it is deferencing a memslot because the translations stored in
the TLB are effectively gfn->hva->hpa.  Obviously KVM ensures that it won't
dereference a memslot that has been deleted/moved, but it's a lot more
subtle than simply holding a lock.

> After all this is super slow path so I wouldn't mind to take the lock
> for some time longer.

Holding the lock doesn't affect this super slow vmx_set_tss_addr(), it
affects everything else that wants slots_lock.  Now, admittedly it's
extremely unlikely userspace is going to do KVM_SET_USER_MEMORY_REGION in
parallel, but that's not the point and it's not why I'm objecting to
holding the lock.

Holding the lock implies protection that is *not* provided.  You and I know
it's not needed for copy_{to,from}_user(), but look how long it's taken us
to get on the same page.  A future KVM developer comes along, sees this
code, and thinks "oh, I need to hold slots_lock to dereference a gfn", and
propagates the unnecessary locking to some other code.

> Or otherwise if you really like the unlock() to
> be earlier I can comment above the unlock:
> 
>   /*
>    * We can unlock before using the HVA only because this KVM private
>    * memory slot will never change until the end of VM lifecycle.
>    */

How about:

	/*
	 * No need to hold slots_lock while filling the TSS, the TSS private
	 * memslot is guaranteed to be valid until the VM is destroyed, i.e.
	 * there is no danger of corrupting guest memory by consuming a stale
	 * gfn->hva lookup.
	 */

  reply index

Thread overview: 82+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-09 14:57 [PATCH v3 00/21] KVM: Dirty ring interface Peter Xu
2020-01-09 14:57 ` [PATCH v3 01/21] vfio: introduce vfio_iova_rw to read/write a range of IOVAs Peter Xu
2020-01-09 14:57 ` [PATCH v3 02/21] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_iova_rw Peter Xu
2020-01-09 14:57 ` [PATCH v3 03/21] KVM: Remove kvm_read_guest_atomic() Peter Xu
2020-01-09 14:57 ` [PATCH v3 04/21] KVM: Add build-time error check on kvm_run size Peter Xu
2020-01-09 14:57 ` [PATCH v3 05/21] KVM: X86: Change parameter for fast_page_fault tracepoint Peter Xu
2020-01-09 14:57 ` [PATCH v3 06/21] KVM: X86: Don't take srcu lock in init_rmode_identity_map() Peter Xu
2020-01-09 14:57 ` [PATCH v3 07/21] KVM: Cache as_id in kvm_memory_slot Peter Xu
2020-01-09 14:57 ` [PATCH v3 08/21] KVM: X86: Drop x86_set_memory_region() Peter Xu
2020-01-09 14:57 ` [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR] Peter Xu
2020-01-19  9:01   ` Paolo Bonzini
2020-01-20  6:45     ` Peter Xu
2020-01-21 15:56   ` Sean Christopherson
2020-01-21 16:14     ` Paolo Bonzini
2020-01-28  5:50     ` Peter Xu
2020-01-28 18:24       ` Sean Christopherson
2020-01-31 15:08         ` Peter Xu
2020-01-31 19:33           ` Sean Christopherson
2020-01-31 20:28             ` Peter Xu
2020-01-31 20:36               ` Sean Christopherson
2020-01-31 20:55                 ` Peter Xu
2020-01-31 21:29                   ` Sean Christopherson [this message]
2020-01-31 22:16                     ` Peter Xu
2020-01-31 22:20                       ` Sean Christopherson
2020-01-09 14:57 ` [PATCH v3 10/21] KVM: Pass in kvm pointer into mark_page_dirty_in_slot() Peter Xu
2020-01-09 14:57 ` [PATCH v3 11/21] KVM: Move running VCPU from ARM to common code Peter Xu
2020-01-09 14:57 ` [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking Peter Xu
2020-01-09 16:29   ` Michael S. Tsirkin
2020-01-09 16:56     ` Alex Williamson
2020-01-09 19:21       ` Peter Xu
2020-01-09 19:36         ` Michael S. Tsirkin
2020-01-09 19:15     ` Peter Xu
2020-01-09 19:35       ` Michael S. Tsirkin
2020-01-09 20:19         ` Peter Xu
2020-01-09 22:18           ` Michael S. Tsirkin
2020-01-10 15:29             ` Peter Xu
2020-01-12  6:24               ` Michael S. Tsirkin
2020-01-14 20:01         ` Peter Xu
2020-01-15  6:50           ` Michael S. Tsirkin
2020-01-15 15:20             ` Peter Xu
2020-01-19  9:09       ` Paolo Bonzini
2020-01-19 10:12         ` Michael S. Tsirkin
2020-01-20  7:29           ` Peter Xu
2020-01-20  7:47             ` Michael S. Tsirkin
2020-01-21  8:29               ` Peter Xu
2020-01-21 10:25                 ` Paolo Bonzini
2020-01-21 10:24             ` Paolo Bonzini
2020-01-11  4:49   ` kbuild test robot
2020-01-11 23:19   ` kbuild test robot
2020-01-15  6:47   ` Michael S. Tsirkin
2020-01-15 15:27     ` Peter Xu
2020-01-16  8:38   ` Michael S. Tsirkin
2020-01-16 16:27     ` Peter Xu
2020-01-17  9:50       ` Michael S. Tsirkin
2020-01-20  6:48         ` Peter Xu
2020-01-09 14:57 ` [PATCH v3 13/21] KVM: Make dirty ring exclusive to dirty bitmap log Peter Xu
2020-01-09 14:57 ` [PATCH v3 14/21] KVM: Don't allocate dirty bitmap if dirty ring is enabled Peter Xu
2020-01-09 16:41   ` Peter Xu
2020-01-09 14:57 ` [PATCH v3 15/21] KVM: selftests: Always clear dirty bitmap after iteration Peter Xu
2020-01-09 14:57 ` [PATCH v3 16/21] KVM: selftests: Sync uapi/linux/kvm.h to tools/ Peter Xu
2020-01-09 14:57 ` [PATCH v3 17/21] KVM: selftests: Use a single binary for dirty/clear log test Peter Xu
2020-01-09 14:57 ` [PATCH v3 18/21] KVM: selftests: Introduce after_vcpu_run hook for dirty " Peter Xu
2020-01-09 14:57 ` [PATCH v3 19/21] KVM: selftests: Add dirty ring buffer test Peter Xu
2020-01-09 14:57 ` [PATCH v3 20/21] KVM: selftests: Let dirty_log_test async for dirty ring test Peter Xu
2020-01-09 14:57 ` [PATCH v3 21/21] KVM: selftests: Add "-c" parameter to dirty log test Peter Xu
2020-01-09 15:59 ` [PATCH v3 00/21] KVM: Dirty ring interface Michael S. Tsirkin
2020-01-09 16:17   ` Peter Xu
2020-01-09 16:40     ` Michael S. Tsirkin
2020-01-09 17:08       ` Peter Xu
2020-01-09 19:08         ` Michael S. Tsirkin
2020-01-09 19:39           ` Peter Xu
2020-01-09 20:42             ` Paolo Bonzini
2020-01-09 22:28             ` Michael S. Tsirkin
2020-01-10 15:10               ` Peter Xu
2020-01-09 16:47 ` Alex Williamson
2020-01-09 17:58   ` Peter Xu
2020-01-09 19:13     ` Michael S. Tsirkin
2020-01-09 19:23       ` Peter Xu
2020-01-09 19:37         ` Michael S. Tsirkin
2020-01-09 20:51       ` Paolo Bonzini
2020-01-09 22:21         ` Michael S. Tsirkin
2020-01-19  9:11 ` Paolo Bonzini

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