Re: [Xen-devel] [PATCH v5 5/6] xen/x86: add PHYSDEVOP_msi_control

[Jan Beulich <JBeulich@xxxxxxxx>]

On 05.08.2019 15:44, Marek Marczykowski-Górecki  wrote:
> On Fri, Jul 19, 2019 at 09:43:26AM +0000, Jan Beulich wrote:
>> On 19.07.2019 11:02, Roger Pau Monné  wrote:
>>> On Fri, Jul 19, 2019 at 08:04:45AM +0000, Jan Beulich wrote:
>>>> On 18.07.2019 18:52, Roger Pau Monné  wrote:
>>>>> On Thu, Jul 18, 2019 at 03:17:27PM +0000, Jan Beulich wrote:
>>>>>> On 18.07.2019 15:46, Roger Pau Monné  wrote:
>>>>>>> In fact I don't think INTx should be enabled when MSI(-X) is disabled,
>>>>>>> QEMU already traps writes to the command register, and it will manage
>>>>>>> INTx enabling/disabling by itself. I think the only check required is
>>>>>>> that MSI(-X) cannot be enabled if INTx is also enabled. In the same
>>>>>>> way both MSI caspabilities cannot be enabled simultaneously. The
>>>>>>> function should not explicitly disable any of the other capabilities,
>>>>>>> and just return -EBUSY if the caller attempts for example to enable
>>>>>>> MSI while INTx or MSI-X is enabled.
>>>>>>
>>>>>> You do realize that pci_intx() only ever gets called for Xen
>>>>>> internally used interrupts, i.e. mainly the serial console one?
>>>>>
>>>>> You will have to bear with me because I'm not sure I understand why
>>>>> it does matter. Do you mean to point out that dom0 is the one in full
>>>>> control of INTx, and thus Xen shouldn't care of whether INTx and
>>>>> MSI(-X) are enabled at the same time?
>>>>>
>>>>> I still think that at least a warning should be printed if a caller
>>>>> tries to enable MSI(-X) while INTx is also enabled, but unless there's
>>>>> a reason to have both MSI(-X) and INTx enabled at the same time (maybe
>>>>> a quirk for some hardware issue?) it shouldn't be allowed on this new
>>>>> interface.
>>>>
>>>> I don't mind improvements to the current situation (i.e. such a
>>>> warning may indeed make sense); I merely stated how things currently
>>>> are. INTx treatment was completely left aside when MSI support was
>>>> introduced into Xen.
>>>
>>> In order to give Marek a more concise reply, would you agree to return
>>> -EBUSY (or some error code) and print a warning message if the caller
>>> attempts to enable MSI(-X) while INTx is also enabled?
>>
>> As to returning an error - I think so, yes. I'm less sure about logging
>> a message.
> 
> I'm trying to get it working and it isn't clear to me what should I
> check for "INTx is also enabled". I assumed PCI_COMMAND_INTX_DISABLE
> bit, but it looks like guest has no control over this bit, even in
> permissive mode.  This means enabling MSI(-X) always fails because guest
> has no way to set PCI_COMMAND_INTX_DISABLE bit before.

Well, the guest has no control, but in order to enable MSI{,-X} I'd
have expected qemu or the Dom0 kernel to set this bit up front. If
that's not the case, then of course neither checking nor logging a
message is appropriate at this point in time. It may be worthwhile
calling out this anomaly then in the description.

Jan
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