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Re: [PATCH 1/4] s390x/pci: use a reserved ID for the default PCI group

From: Pierre Morel
Subject: Re: [PATCH 1/4] s390x/pci: use a reserved ID for the default PCI group
Date: Fri, 3 Dec 2021 10:07:03 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0

On 12/3/21 03:25, Matthew Rosato wrote:
On 12/2/21 6:06 PM, Halil Pasic wrote:
On Thu, 2 Dec 2021 12:11:38 -0500
Matthew Rosato <mjrosato@linux.ibm.com> wrote:

What happens if we migrate a VM from old to new QEMU? Won't the guest be
able to observe the change?

Yes, technically --  But # itself is not really all that important, it
is provided from CLP Q PCI FN to be subsequently used as input into Q
PCI FNGRP -- With the fundamental notion being that all functions that
share the same group # share the same group CLP info.  Whether the
number is, say, 1 or 5 doesn't matter so much.

However..  0xF0 and greater are the only values reserved for hypervisor
use.  By using 0x20 we run the risk of accidentally conflating simulated
devices and real hardware, hence the desire to change it.

Is your concern about a migrated guest with a virtio device trying to do
a CLP QUERY PCI FNGRP using 0x20 on a new QEMU?  I suppose we could
modify 'clp_service_call, case CLP_QUERY_PCI_FNGRP' to silently catch
simulated devices trying to use something other than the default group,

if ((pbdev->fh & FH_SHM_EMUL) &&
      (pbdev->zpci_fn.pfgid != ZPCI_DEFAULT_FN_GRP)) {
          /* Simulated device MUST have default group */
    pbdev->zpci_fn.pfgid = ZPCI_DEFAULT_FN_GRP;
    group = s390_group_find(ZPCI_DEFAULT_FN_GRP);

What do you think?

Another option, and in my opinion the cleaner one would be to tie this
change to a new machine version. That is if a post-change qemu is used
in compatibility mode, we would still have the old behavior.

What do you think?

The problem there is that the old behavior goes against the architecture (group 0x20 could belong to real hardware) and AFAIU assigning this new behavior only to a new machine version means we can't fix old stable QEMU versions.

Also, wait a minute -- migration isn't even an option right now, it's blocked for zpci devices, both passthrough and simulated (see aede5d5dfc5f 's390x/pci: mark zpci devices as unmigratable') so I say let's just move to a proper default group now before we potentially allow migration later.

Looks right to me.

Pierre Morel
IBM Lab Boeblingen

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