Re: [Xen-devel] [PATCH v2] interrupts: allow guest to set and clear MSI-X mask bit

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.08.13 at 18:03, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 05/08/13 12:49, Jan Beulich wrote:
>> But a proper solution would of course be preferred. That may involve
>> notifying Xen of the reset from the PF driver.
> 
> That is making the assumption that the PF driver is even aware that the
> reset has occurred.  If the PF driver can get at the reset information,
> I still cant see Xen specific hooks being accepted upstream.

You sort of contradict your earlier statement here that "the VF
requests the PF to reset itself" (where you probably meant "it",
as the VF is hardly allowed to cause a reset of the PF): Such an
operation should imo be visible to the PF driver.

And then, even if the reset is being done, shouldn't the interrupt
setup occur _after_ the reset? Which would make Xen clear the
flag...

> Fundamentally, given that only the VM is in a position to actually know
> about the state of the mask bit (As we cant possibly have Xen polling
> for the state), then the guest has to have access to the mask bit.

No, that continues to not be an option as long as Xen uses the mask
bit itself. Once again, the obvious immediate solution would appear
to be to carry out the write with the OR of both the guest intended
and the hypervisor required mask bits, thus allowing the bit to get
cleared immediately if Xen doesn't need it to be set (and the clearing
would happen subsequently if Xen needs the flag to be set at the
point of the guest write). This requires associating back the table
entry to the IRQ in question (if any), in order to look at
irq_desc->msi_desc->msi_attrib.masked.

Jan


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