linux-kernel - Re: [PATCH 10/14] KVM: x86: Further clarify the logic and comments for toggling log dirty

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Message-ID: <YC6SjgYTi+Dr1l0d@google.com>
Date:   Thu, 18 Feb 2021 08:15:10 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>
Subject: Re: [PATCH 10/14] KVM: x86: Further clarify the logic and comments
 for toggling log dirty

On Thu, Feb 18, 2021, Paolo Bonzini wrote:
> On 13/02/21 01:50, Sean Christopherson wrote:
> > 
> > -	 * Nothing to do for RO slots or CREATE/MOVE/DELETE of a slot.
> > -	 * See comments below.
> > +	 * Nothing to do for RO slots (which can't be dirtied and can't be made
> > +	 * writable) or CREATE/MOVE/DELETE of a slot.  See comments below.
> >  	 */
> >  	if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
> >  		return;
> > +	/*
> > +	 * READONLY and non-flags changes were filtered out above, and the only
> > +	 * other flag is LOG_DIRTY_PAGES, i.e. something is wrong if dirty
> > +	 * logging isn't being toggled on or off.
> > +	 */
> > +	if (WARN_ON_ONCE(!((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)))
> > +		return;
> > +
> 
> What about readonly -> readwrite changes?

Not allowed without first deleting the memslot.  See commit 75d61fbcf563 ("KVM:
set_memory_region: Disallow changing read-only attribute later").  RW->RO is
also not supported.

	if (!old.npages) {
		change = KVM_MR_CREATE;
		new.dirty_bitmap = NULL;
		memset(&new.arch, 0, sizeof(new.arch));
	} else { /* Modify an existing slot. */
		if ((new.userspace_addr != old.userspace_addr) ||
		    (new.npages != old.npages) ||
		    ((new.flags ^ old.flags) & KVM_MEM_READONLY))
			return -EINVAL;