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

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Date:   Thu, 18 Feb 2021 17:56:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 18/02/21 17:15, Sean Christopherson wrote:
> 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;
> 

Right you are, thanks.  Then the warning would catch this quick.  I 
queued 10 and 11 too, and will reply on 12 now that I looked at it more 
closely.

Paolo