Re: [Xen-devel] [PATCH v5] x86/emulate: Send vm_event from emulate

From: Alexandru Stefan ISAILA <aisaila@bitdefender.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Wei Liu <wl@xen.org>,
	"kevin.tian@intel.com" <kevin.tian@intel.com>,
	"tamas@tklengyel.com" <tamas@tklengyel.com>,
	"jun.nakajima@intel.com" <jun.nakajima@intel.com>,
	"rcojocaru@bitdefender.com" <rcojocaru@bitdefender.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Tim Deegan <tim@xen.org>, PaulDurrant <Paul.Durrant@citrix.com>,
	"suravee.suthikulpanit@amd.com" <suravee.suthikulpanit@amd.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	"brian.woods@amd.com" <brian.woods@amd.com>,
	RogerPauMonne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v5] x86/emulate: Send vm_event from emulate
Date: Tue, 2 Jul 2019 08:10:56 +0000	[thread overview]
Message-ID: <3f615acc-01d7-3c5c-13ab-42df23cb2172@bitdefender.com> (raw)
In-Reply-To: <193216ae-a9f5-4d83-4acd-c6786bb2ccbb@suse.com>

On 02.07.2019 11:01, Jan Beulich wrote:
> On 02.07.2019 09:58, Alexandru Stefan ISAILA wrote:
>>
>>
>> On 01.07.2019 18:53, Jan Beulich wrote:
>>> On 01.07.2019 17:36, Alexandru Stefan ISAILA wrote:
>>>> On 01.07.2019 17:55, Jan Beulich wrote:
>>>>> On 01.07.2019 16:45, Alexandru Stefan ISAILA wrote:
>>>>>> On 01.07.2019 16:13, Jan Beulich wrote:
>>>>>>>>>> On 04.06.19 at 13:49, <aisaila@bitdefender.com> wrote:
>>>>>>>> +    if ( !req.u.mem_access.flags )
>>>>>>>> +        return false; /* no violation */
>>>>>>>
>>>>>>> How is the "false" here (I think this is the one the description talks
>>>>>>> about) matching up with the various other ones in the function?
>>>>>>
>>>>>> There should be no event if there is no access violation. So in this
>>>>>> case the emulation is continued as expected.
>>>>>
>>>>> But this doesn't answer my question: You use "false" as return value
>>>>> to indicate different things. Only the one here means "no access
>>>>> violation".
>>>>
>>>> Sorry about that, since this will remain the only return false apart
>>>> form the main one (return monitor_traps()), false  = event was not sent
>>>> and true = event was sent.
>>>>
>>>> I understand that you are asking about the scenario when there was a
>>>> violation and the event was not sent. Then I can issue a domain_crash()
>>>> as that is potentially a big issue.
>>>>
>>>> I hope I got that correctly.
>>>
>>> I don't get the impression that you did. I count a total of four
>>> "return false" in the function, only one of which explicitly means
>>> "no access violation" (others may have that meaning implicitly). Let's
>>> take the p2m_get_mem_access() failure case as an example: What I don't
>>> understand is why this case and the "no access violation" one are both
>>> meant to be treated the same.
>>
>> Right, at the moment, false means that emulation should continue and
>> true means that emulation should stop. If it is a must that I return
>> different errors I will change that in the next version but in the way
>> that the code is right now they will be treated the same way.
> 
> Again - it's not a requirement. It depends on whether the behavior is
> intended to be that way. If it is, it should be clarified in the
> description or maybe better in a code comment. But to me, without such
> a clarification, it doesn't look like it should be that way.
> 

Short answer is yes, the behavior is the same and it does not need any 
differentiation, I will clarify this in a comment and in the commit 
comment, sorry for misunderstanding the first comment.

Regards,
Alex
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