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

[Markus Armbruster <armbru@redhat.com>]

Cao jin <caoj.fnst@cn.fujitsu.com> writes:

> 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.

You got it.

> 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?

While I don't particularly like additional configuration knobs, I like
the "try to use MSI, fall back to INTx silently" behavior even less.
I'd welcome such a property.  Can guarantee the respective maintainers
will agree with me, of course.