On 5/10/19 7:47 AM, Cornelia Huck wrote:
On Wed, 8 May 2019 11:22:07 +0200
Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:
For the NOOP its clearly stated that it does not start a data transfer.
If we pin the CDA, it could then eventually be the cause of errors if
the address indicated by the CDA is not accessible.
The NOOP is a particular CONTROL operation for which no data is transfered.
Other CONTROL operation may start a data transfer.
I've just looked at the documentation again.
The Olde Common I/O Device Commands document indicates that a NOOP
simply causes channel end/device end.
The PoP seems to indicate that the cda is always checked (i.e. does it
point to a valid memory area?), but I'm not sure whether the area that
is pointed to is checked for accessibility etc. as well, even if the
command does not transfer any data.
Has somebody tried to find out what happens on Real Hardware(tm) if you
send a command that is not supposed to transfer any data where the cda
points to a valid, but not accessible area?
Hrm... The CDA itself? I don't think so. Since every CCW is converted
to an IDAL in vfio-ccw, we guarantee that it's pointing to something
valid at that point.
So, I hacked ccwchain_fetch_direct() to NOT set the IDAL flag in a NOP
CCW, and to leave the CDA alone. This means it will still contain the
guest address, which is risky but hey it's a test system. :) (I
offline'd a bunch of host memory too, to make sure I had some
unavailable addresses.)
In my traces, the non-IDA NOP CCWs were issued to the host with and
without the skip flag, with zero and non-zero counts, and with zero and
non-zero CDAs. All of them work just fine, including the ones who's
addresses fall into the offline space. Even the combination of no skip,
non-zero count, and zero cda.
I modified that hack to do the same for a known invalid control opcode,
and it seemed to be okay too. We got an (expected) invalid command
before we noticed any problem with the provided address.
In general, I think doing the translation (and probably already hitting
errors there) is better than sending down a guest address.
I mostly agree, but I have one test program that generates invalid GUEST
addresses with its NOP CCWs, since it doesn't seem to care about whether
they're valid or not. So any attempt to pin them will end badly, which
is why I call that opcode out in ccw_does_data_transfer(), and just send
invalid IDAWs with it.
