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

From: Peter Xu <peterx@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "James Hogan" <jhogan@kernel.org>,
	"Paul Mackerras" <paulus@ozlabs.org>,
	"Christian Borntraeger" <borntraeger@de.ibm.com>,
	"Janosch Frank" <frankja@linux.ibm.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Marc Zyngier" <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Philippe Mathieu-Daudé" <f4bug@amsat.org>,
	kvm@vger.kernel.org, "David Hildenbrand" <david@redhat.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"Cornelia Huck" <cohuck@redhat.com>,
	linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm-ppc@vger.kernel.org, "Vitaly Kuznetsov" <vkuznets@redhat.com>,
	kvmarm@lists.cs.columbia.edu, "Jim Mattson" <jmattson@google.com>
Subject: Re: [PATCH v4 16/19] KVM: Ensure validity of memslot with respect to kvm_get_dirty_log()
Date: Thu, 6 Feb 2020 17:03:55 -0500	[thread overview]
Message-ID: <20200206220355.GH700495@xz-x1> (raw)
In-Reply-To: <20200114182506.GF16784@linux.intel.com>

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,

-- 
Peter Xu