Re: [qemu-s390x] [PATCH RFC 2/2] vfio-ccw: support for halt/clear subchannel

[Halil Pasic <pasic@linux.ibm.com>]

On 06/07/2018 11:54 AM, Cornelia Huck wrote:
> Hm, I think we need to be more precise as to what scsw we're talking
> about. Bad ascii art time:
> 
> --------------
> |   scsw(g)  |  ssch
> --------------   |
>                   |                                       guest
> --------------------------------------------------------------
>                   |                                        qemu
> --------------   v
> |   scsw(q)  | emulate
> --------------   |
>                   |
> --------------   v
> |   scsw(r)  | pwrite()
> --------------   |
>                   |
> --------------------------------------------------------------
>                   |                                        vfio
>                   v
>                  ssch
>                   |
> --------------------------------------------------------------
>                   |                                    hardware
> --------------   v
> |   scsw(h)  | actually do something
> --------------
> 
> The guest issues a ssch (which gets intercepted; it won't get control
> back until ssch finishes with a cc set.) scsw(g) won't change, unless
> the guest does a stsch for the subchannel on another vcpu, in which
> case it will get whatever information qemu holds in scsw(q) at that
> point in time.

(1) I think BQL make other cpu or not other kind of the same. We will
effectively start processing the stsch in QEMU after we are done
with the ssch in QEMU.

> 
> When qemu starts to emulate the guest's ssch, it will set the start
> function bit in the fctl field of scsw(q). It then copies scsw(q) to
> scsw(r) in the vfio region.
> 

(2) This is architecturally wrong AFAIK. The fctl bit is supposed to be set on
cc 0. But because of (1) this might not be a observable by the guest --
we can fix it up.

(3)IMHO scsw(r) is not a real scsw as defined by the architecture but
a strange communication structure (not) defined vfio-ccw.

> The vfio code will then proceed to call ssch on the real subchannel.
> This is the first time we get really asynchronous, as the ssch will
> return with cc set and the start function will be performed at some
> point in time. If we would do a stsch on the real subchannel, we would
> see that scsw(h) now has the start function bit set.
> 

(4) I guess only if cc 0.

> Currently, we won't return back up the chain until we get an interrupt
> from the hardware, at which time we update the scsw(r) from the irb.
> This will propagate into the scsw(q). At the time we finish handling
> the guest's ssch and return control to it, we're all done and if the
> guest does a stsch to update its scsw(g), it will get the current
> scsw(q) which will already contain the scsw from the interrupt's irb
> (indicating that the start function is already finished).
> 
> Now let's imagine we have a future implementation that handles actually
> performing the start on the hardware asynchronously, i.e. it returns
> control to the guest without the interrupt having been posted (let's
> say that it is a longer-running I/O request). If the guest now did a
> stsch to update scsw(g), it would get the current state of scsw(q),
> which would be "start function set, but not done yet".

(5) AFAIK this is how the current implementation works. We don't wait
for the I/O interrupt on the host to present a cc to the guest for it's
ssch.

> 
> If the guest now does a hsch, it would trap in the same way as the ssch
> before. When qemu gets control, it adds the halt bit in scsw(q) (which
> is in accordance with the architecture).

(7) Again it's when is fctl set according to the architecture...

> My proposal is to do the same
> copying to scsw(r) again, which would mean we get a request with both
> the halt and the start bit set.

(8) IMHO when receiving the 'request' we are and should be in instruction
context -- opposed to basic io function context. So we should not set fctl
before we know what will our guest cc be. But since scsw(r) is not a real
scsw it is just strange.

> The vfio code now needs to do a hsch
> (instead of a ssch). The real channel subsystem should figure this out,
> as we can't reliably check whether the start function has concluded
> already (there's always a race window).
> 

(9) Yes we can't tell for sure if the start function is still being performed
by the stuff below.

Regards,
Halil

> For csch, things are a bit different (which the code posted here did
> not take into account). The qemu emulation of csch needs to clear any
> start/halt bits in scsw(q) when setting the clear bit there, and
> therefore scsw(r) will only have the clear bit set in that case. We
> still should do an unconditional csch for the same reasons as above;
> the hardware will do the same things (clearing start/halt, setting
> clear) in the scsw(h).
> 
> Congratulations, you've reached the end:)  I hope that was helpful and
> not too confusing.
>