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

From: Marcel Apfelbaum
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH RFC v2 1/2] Add param Error** to msi_init() & modify the callers
Date: Thu, 3 Mar 2016 13:19:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

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 

        This is a safety measure to avoid breaking platforms which should 
        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

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.

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" 


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

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