Re: [Qemu-devel] [PATCH v4 5/5] Add param Error ** for msi_init()

[Cao jin <caoj.fnst@cn.fujitsu.com>]

Hi Markus,
     sorry for replying so late, I am stucked by other tasks for a while.

On 04/12/2016 07:50 PM, Markus Armbruster wrote:
>
> Examine how it uses msi_init().  That's how we give a PCI device
> capability MSI.  If the device model treats msi_init() failure as fatal,
> it doesn't have a non-MSI variant.
>
> Some device models let the user ask for MSI with a property, and make
> the msi_init() call depend on the property.  The property is commonly
> called "msi" or "use_msi", with values "on" and "off".
>
> msi=off then works as expected: you get the variant without MSI.
>
> The meaning of msi=on depends on how msi_init() failure is handled.  If
> it's fatal, then msi=on works as expected: you get the variant with MSI.
> But if it's not fatal, then you may or may not get it, which I consider
> a grossly misleading user interface.
>
> To clean this up, we could add msi=auto, and move the non-fatal behavior
> from msi=on to msi=auto.
>
> Same for MSI-X with msix_init() and property "msix".
>
Let me try to understand your meaning, correct me pls if I am wrong:

replace the msi property type from bit/int to enum OnOffAuto(it seems 
need a tiny surgery for device structure), and default to auto. Then 
process going to look like this:

(msi property = auto) means enable msi by default. If msi_init fail, we 
switch to the non-MSI variant; If msi_init success, we got msi variant.

Another condition I want to mention, ahci, pvscsi & vmxnet3 don`t have 
msi property, and when msi_init fail, they all will use intx. One thing 
need to be confirmed: whether they need a msi property or not?

-- 
Yours Sincerely,

Cao jin