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Re: [Qemu-block] [Qemu-devel] [PATCH RFC v2 1/2] Add param Error** to ms

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH RFC v2 1/2] Add param Error** to msi_init() & modify the callers
Date: Thu, 03 Mar 2016 16:03:16 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

"Michael S. Tsirkin" <address@hidden> writes:

> On Thu, Mar 03, 2016 at 01:19:09PM +0200, Marcel Apfelbaum wrote:
>> On 03/03/2016 12:45 PM, Michael S. Tsirkin wrote:
>> >On Thu, Mar 03, 2016 at 12:12:27PM +0200, Marcel Apfelbaum wrote:
>> >>>>+int msi_init(struct PCIDevice *dev, uint8_t offset, unsigned int 
>> >>>>nr_vectors,
>> >>>>+             bool msi64bit, bool msi_per_vector_mask, Error **errp)
>> >>>>  {
>> >>>>      unsigned int vectors_order;
>> >>>>-    uint16_t flags;
>> >>>>+    uint16_t flags; /* Message Control register value */
>> >>>>      uint8_t cap_size;
>> >>>>      int config_offset;
>> >>>>
>> >>>>      if (!msi_supported) {
>> >>>>+        error_setg(errp, "MSI is not supported by interrupt 
>> >>>>controller");
>> >>>>          return -ENOTSUP;
>> >>>
>> >>>First failure mode: board does not support MSI (-ENOTSUP).
>> >>>
>> >>>Question to the PCI guys: why is this even an error?  A device with
>> >>>capability MSI should work just fine in such a board.
>> >>
>> >>Hi Markus,
>> >>
>> >>Adding Jan Kiszka, maybe he can help.
>> >>
>> >>That's a fair question. Is there any history for this decision?
>> >>The board not supporting MSI has nothing to do with the capability being 
>> >>there.
>> >>The HW should not change because the board doe not support it.
>> >>
>> >>The capability should be present but not active.
>> >
>> >Digging in git log will tell you why we have the msi_supported flag:
>> >
>> >commit 02eb84d0ec97f183ac23ee939403a139e8849b1d ("qemu/pci: MSI-X support 
>> >functions")
>> >
>> >    This is a safety measure to avoid breaking platforms which should 
>> > support
>> >    MSI-X but currently lack this in the interrupt controller emulation.
>> >
>> >in other words, on some platforms we must hide msi support from devices
>> >because otherwise guests will try to use it, and our emulation is
>> >incomplete.
>> OK, thanks. So the flag should be "msi_broken" or 
>> "msi_present_but_not_implemented" and not
>> "msi_supported" that leads (at least me) to the assumption that some 
>> platform *does not support msi*
>> rather than it supports it, but we don't emulate it.

I agree the name is badly misleading for this role.

Now let me see how this contraption actually works.  msi_supported is
global, initialized to false, and becomes globally true when

1. certain MSI-capable interrupt controllers realize: "apic",
  "kvm-apic" if kvm_has_gsi_routing(), "xen-apic", "arm-gicv2m",
  "openpic" models 1 and 2, "kvm-openpic" models 1 and 2

2. "s390-pcihost" class-initializes

3. machine "spapr-machine" initializes


* "Global is problematic.  What if a board has more than one interrupt
  controller?  What if one of them sets msi_supported, but the other one
  is of the kind Michael described, i.e. guests know it has MSI, but our
  emulation doesn't actually work?

* "Initialize to false" is problematic.  We don't clear msi_supported
  when we have a broken interrupt controler, we set it when we have a
  working one.  The consequence is that boards with non-MSI interrupt
  controllers are treated just like boards with broken interrupt

  Here's  how msi_supported is documented:

    /* Flag for interrupt controller to declare MSI/MSI-X support */
    bool msi_supported;

  This is matches how the code works.  However, it contradicts the
  commit message Michael quoted.  The most plausible explanation is that
  the commit is flawed.

* Class-initialize (2.) looks wrong to me.  msi_supported becomes true
  when QOM type "s390-pcihost" is created, regardless of whether
  instances get created, let alone used.

* I'm not sure about 3., but the spapr guys can worry about that.

>> >And the conclusion from that is that for msi_init to fail silently is
>> >at the moment the right thing to do.
>> But this is not the only thing we do, we are modifying the PCI devices. We 
>> could fail starting the VM
>> if a device supporting MSI is added on a platform with broken msi, but this 
>> will prevent us to use
>> assigned devices. Emulated devices should be created with a specific 
>> "msi=off" flag.
>> Thanks,
>> Marcel
> That will just break a bunch of valid configurations, for no real
> benefit to users.

I disagree, strongly.

If I ask for msi=on, then QEMU should either give it to me or fail, not
silently "correct" my configuration.

Of course, if we have msi_supported = false for everybody and its dog
whether it's really needed or not, "or fail" will indeed reject "a bunch
of valid configurations".  We "compensate" by silently messing with the
user's configuration.  That's shoddy workmanship, sorry.

We should instead have msi_supported = false only when it's actually
needed, and thus reject exactly the configurations that won't work.

>> >
>> >The only other reason for it to fail is pci config space corruption,
>> >this probably never happens in practice.
>> >

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