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Re: [Qemu-devel] [PATCH 07/10] target-s390x: enable fully implemented fa

From: Aurelien Jarno
Subject: Re: [Qemu-devel] [PATCH 07/10] target-s390x: enable fully implemented facilities
Date: Tue, 26 May 2015 08:02:16 +0200
User-agent: Mutt/1.5.23 (2014-03-12)

On 2015-05-25 23:47, Alexander Graf wrote:
> On 25.05.15 23:13, Aurelien Jarno wrote:
> > On 2015-05-25 23:04, Alexander Graf wrote:
> >>
> >>
> >> On 25.05.15 23:02, Aurelien Jarno wrote:
> >>> On 2015-05-25 22:39, Alexander Graf wrote:
> >>>>
> >>>>
> >>>> On 25.05.15 01:47, Aurelien Jarno wrote:
> >>>>> Cc: Alexander Graf <address@hidden>
> >>>>> Cc: Richard Henderson <address@hidden>
> >>>>> Signed-off-by: Aurelien Jarno <address@hidden>
> >>>>
> >>>> Shouldn't this get populated based on the selected -cpu type?
> >>>
> >>> In the long term yes, but given we only implement one CPU type (or
> >>> rather none) in TCG mode, we can consider that's already the case.
> >>
> >> There are patches coming from IBM to at least add a list of a good
> >> number of s390x cpu types. I'd really like to make use of that and have
> >> actual CPU types selectable.
> > 
> > I guess they are for the KVM mode. Do they provide the corresponding 
> > facilities list? Probably otherwise that doesn't really differentiate
> > various CPUs. Please make sure of that when reviewing these patches.
> I could definitely use help on review - it's probably my weakest point ;).
> >> At least let's move towards that model. So the code in question should
> >> take the facility capabilities from the first cpu object (or the class?)
> >> for example and we bump it to the currently supported feature set in there.
> > 
> > Yes, that would work for STFL/STFLE, though we should have a list of
> > facilities implemented by TCG so we can mask out the non-implemented
> > facilities. This basically corresponds to the informations provided by
> > the current patch.
> Ah, so you consider the current list the "these are the features TCG
> knows about" list?
> > That said that won't work for actually disabling the corresponding
> > instructions as we don't have a 1 to 1 mapping between the facilities
> > and the group of instructions. Anyway we don't even check that right
> > now.
> I agree, but the TCG code annotates which facility each opcode belongs
> to which means actually limiting it should become trivial. That's really
> all I'm asking for - I want to see the light at the end of the tunnel ;).

It's trivial doing so for the facilities annotated in TCG. Just that
they don't match one to one with the facilities bits in STFL/STFLE. Some
bits enable multiple facilities and QEMU has also grouped some
facilities together. Also some bits do not actually concern instructions
but rather MMU features. Some other gives additional properties to a
facility: some facilities might be present by disabled, some other might 
have a slow or fast implementation.

We therefore need a conversion function before being able to do that,
and we need to know which format IBM is going to provide in their
patches: list of facilities or STFL/STFLE bits?

Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net

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