Re: [Xen-devel] [v4][PATCH 11/19] tools: introduce some new parameters to set rdm policy

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

> > > > > > > If I'm correct, then #3 means it's not possible to have devices for a
> > > > > > > domain *default* to strict, but to be relaxed in individual
> > > > > > > instances.
> > > > > > > If you had five devices you wanted strict, and only one device you
> > > > > > > wanted to be relaxed (because you knew it didn't matter), you'd have
> > > > > > > to set reserved=strict for all the other devices, rather than just
> > > > > > > being able to set the domain setting to strict and set
> > > > > > > reserve=relaxed
> > > > > > > for the one.
> > > > > > >
> > > > > > > I think that both violates the principle of least surprise, and is
> > > > > > > less useful.
> > > > So what's you idea to follow our requirement?
> > > So consider the following config snippet:
> > >
> > > ---
> > > rdm="reserve=relaxed"
> > >
> > > pci=['01:00.1,msitranslate=1']
> > > ----
> > >
> > > What should the policy for that device be?
> > >
> > > According to your policy document, it seems to me like it should be
> > > "relaxed", since the domain default* is set to "relaxed" and nothing
> > Why? "strict" should be in this case.
> OK, I think I see where the problem is.  I had expected the domain-wide
> setting to be a default which was overridden by per-device policies (see
> pci_permissive and friends).  So when I saw "global default RDM policy"
We knew this behavior but we'd like to take a different consideration in 
this case.
> confirmation bias caused me to interpret it as what I expected to see --
> the domain setting as the default, which the local setting could override.
>
> I see now that in your documentation you consistently talk about two
> different policies, each of which have their own defaults, and that the
> effective permissions for a device end up being the intersection of the
> two (i.e., only relaxed of both are relaxed; strict under all other
> circumstances).
>
> > Why are you saying this is not our expectation? Just let me pick up that
> > description *again*,
> >
> > "Default per-device RDM policy is 'strict', while default global RDM
> > policy is 'relaxed'. When both policies are specified on a given region,
> > 'strict' is always preferred."
> Look, if I haven't understood what you meant by the exact same words the
> first 4 times I read it, simply repeating the same exact words is not
> going to be helpful.  Ideally you need to try go understand where my
> misunderstanding is coming from and explain where I've misunderstood
> something; or, at least you need to try to use different words, or
> explain how the words you're using apply to the given situation.
From my point of view, I already replied this previously by quoting 
part of the patch head description. As you know this revision is already 
marked as v4 and although I admit some code implementations still need a 
further review, at least our policy should already acknowledged right 
now unless this is really wrong. But in our case, looks you're 
concerning our mechanism is not expected to you. So
> > > This interface doesn't make any sense to me.  Why, if the "global
> > If you have any objection to our solution, and if you can't find any
> > reasonable answer from our design, just please ping Jan or Kevin because
just do it to make this clear to us. And then, whatever, I'm going be 
fine to step next.
> > I'm really not that person who can address this kind of change at this
> > point in this high level.
> And you have no idea why that design was chosen; you're just doing what
Certainly I have my own understanding with this issue. But

> you're told?
in high level I have to say Yes. If you really read that v2 design and 
its associated discussion, you should notice I didn't put any response 
right there.
> I was involved in the design discussion, and from the very beginning I
> probably saw your plan but misunderstood it.  I wouldn't be surprised if
> some others didn't quite understand what they were agreeing to.
Again, I didn't walk into v2 design. So here I don't want to bring any 
confusion to you just with my reply.
> This way of doing things is different than the way we do it with most
> other options relating to pci devices (e.g., pci_permissive,
> pci_msitranslate, pci_sieze, &c).  All of those options use a "default"
> semantic: the domain-wide setting takes effect only if it's not set
> locally.  If the syntax looks the same but the semantics is different,
> many people will be confused.  If we're going to have the domain-wide
> policy override the per-device policy, then the naming should make that
> clear; for instance, "override=(strict|relaxed|none)", or
> "strict_override=(1|0)".
>
> I don't happen to think these "override" semantics are actually going to
> turn out to be that useful; I do think a "default" semantic would be
> useful.  But I'd be content if the name of the current setting were
> switched to "override" to make the semantics more clear.  We can always
> add in "default" at some later point if we really want.
Just as I said you'd better ping Jan or Kevin to make a point.

Thanks
Tiejun


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