linux-kernel - Re: [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP

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Date:   Fri, 31 Jan 2020 17:16:37 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christophe de Dinechin <dinechin@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Kevin <kevin.tian@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v3 09/21] KVM: X86: Don't track dirty for
 KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]

On Fri, Jan 31, 2020 at 01:29:28PM -0800, Sean Christopherson wrote:
> 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.

At least for a user memory slot, we "need to hold slots_lock to
dereference a gfn" (or srcu), right?

You know I'm suffering from a jetlag today, I thought I was still
fine, now I start to doubt it. :-)

> 
> > 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.
> 	 */

Sure for this.

-- 
Peter Xu