Subject: Re: [PATCH v4 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log()

From: Sean Christopherson <[email-masked]>
Date: Fri, 7 Feb 2020 10:52:47 -0800

On Thu, Feb 06, 2020 at 05:03:55PM -0500, Peter Xu wrote:
> On Tue, Jan 14, 2020 at 10:25:07AM -0800, Sean Christopherson wrote:
> > On Tue, Dec 24, 2019 at 01:19:30PM -0500, Peter Xu wrote:
> > > On Tue, Dec 17, 2019 at 12:40:38PM -0800, Sean Christopherson wrote:
> > > > +int kvm_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log,
> > > > +		      int *is_dirty, struct kvm_memory_slot **memslot)
> > > >  {
> > > >  	struct kvm_memslots *slots;
> > > > -	struct kvm_memory_slot *memslot;
> > > >  	int i, as_id, id;
> > > >  	unsigned long n;
> > > >  	unsigned long any = 0;
> > > >  
> > > > +	*memslot = NULL;
> > > > +	*is_dirty = 0;
> > > > +
> > > >  	as_id = log->slot >> 16;
> > > >  	id = (u16)log->slot;
> > > >  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_USER_MEM_SLOTS)
> > > >  		return -EINVAL;
> > > >  
> > > >  	slots = __kvm_memslots(kvm, as_id);
> > > > -	memslot = id_to_memslot(slots, id);
> > > > -	if (!memslot->dirty_bitmap)
> > > > +	*memslot = id_to_memslot(slots, id);
> > > > +	if (!(*memslot)->dirty_bitmap)
> > > >  		return -ENOENT;
> > > >  
> > > > -	n = kvm_dirty_bitmap_bytes(memslot);
> > > > +	kvm_arch_sync_dirty_log(kvm, *memslot);
> > > 
> > > Should this line belong to previous patch?
> > 
> > No.
> > 
> > The previous patch, "KVM: Provide common implementation for generic dirty
> > log functions", is consolidating the implementation of dirty log functions
> > for architectures with CONFIG_KVM_GENERIC_DIRTYLOG_READ_PROTECT=y.
> > 
> > This code is being moved from s390's kvm_vm_ioctl_get_dirty_log(), as s390
> > doesn't select KVM_GENERIC_DIRTYLOG_READ_PROTECT.  It's functionally a nop
> > as kvm_arch_sync_dirty_log() is empty for PowerPC, the only other arch that
> > doesn't select KVM_GENERIC_DIRTYLOG_READ_PROTECT.
> > 
> > Arguably, the call to kvm_arch_sync_dirty_log() should be moved in a
> > separate prep patch.  It can't be a follow-on patch as that would swap the
> > ordering of kvm_arch_sync_dirty_log() and kvm_dirty_bitmap_bytes(), etc...
> > 
> > My reasoning for not splitting it to a separate patch is that prior to this
> > patch, the common code and arch specific code are doing separate memslot
> > lookups via id_to_memslot(), i.e. moving the kvm_arch_sync_dirty_log() call
> > would operate on a "different" memslot.   It can't actually be a different
> > memslot because slots_lock is held, it just felt weird.
> > 
> > All that being said, I don't have a strong opinion on moving the call to
> > kvm_arch_sync_dirty_log() in a separate patch; IIRC, I vascillated between
> > the two options when writing the code.  If anyone wants it to be a separate
> > patch I'll happily split it out.
> 
> (Sorry to respond so late)
> 
> I think the confusing part is the subject, where you only mentioned
> the memslot change.  IMHO you can split the change to make it clearer,
> or at least would you mind mention that kvm_arch_sync_dirty_log() move
> in the commit message?  Thanks,

I'll add a few paragraphs to the changelog.  Splitting it out still feels
weird.

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