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Re: [Xen-devel] [PATCH 03/18] xen/arm: Save GIC and virtual timer context when the domain suspends

(+ Andre)

On 11/12/18 5:42 PM, Mirela Simonovic wrote:
Hi Julien,

On Mon, Nov 12, 2018 at 6:00 PM Julien Grall <julien.grall@xxxxxxx> wrote:

On 11/12/18 4:52 PM, Mirela Simonovic wrote:
Hi Julien,


Thanks for the feedback.

On Mon, Nov 12, 2018 at 4:36 PM Julien Grall <julien.grall@xxxxxxx> wrote:

Hi Mirela,

On 11/12/18 11:30 AM, Mirela Simonovic wrote:
GIC and virtual timer context must be saved when the domain suspends.

Please provide the rationale for this. Is it required by the spec?

This is required for GIC because a guest leaves enabled interrupts in
the GIC that could wake it up, and on resume it should be able to
detect which interrupt woke it up. Without saving/restoring the state
of GIC this would not be possible.

I am confused. In patch #10, you save the GIC host because the GIC can
be powered-down. Linux is also saving the GIC state. So how the
interrupt can come up if the GIC is powered down?

After Xen (or Linux in the config without Xen) hands over the control
to EL3 on suspend (calls system suspend PSCI to EL3), it leaves
enabled interrupts that are its wake-up sources. Saving a GIC state
only means that its current configuration will be remembered somewhere
in data structures, but the configuration is not changed on suspend.
Everything that happens with interrupt configuration from this point
on is platform specific. Now there are 2 options: 1) EL3 will never
allow CPU0 to be powered down and the wake-up interrupt will indeed
propagate via GIC;
or 2) CPU0 will be powered down and the GIC may be
powered down as well, so an external help is needed to deal with
wake-up and resume (e.g. in order to react to a wake-up interrupt
while the GIC is down, and power up CPU0). This external help is
provided by some low-power processor outside of the Cortex-A cluster.

So the platform firmware is responsible for properly configuring a
wake-up path if GIC goes down. This is commonly handled by EL3
communicating with low-power processor. When the GIC power comes up,
the interrupt triggered by a peripheral is still active and the
software on Cortex-A cluster should be able to observe it once the GIC
state is restored, i.e. interrupts get enabled at GIC.

Thank you for the explanation. Now the question is why can't we reset at least the GIC CPU interface?

AFAIU, the guest should restore them in any case. The only things we need to know is the interrupt was received for a given guest. We can then queue it and wake-up the domain.

This seems to fit with the description on top of gic_dist_save() in Linux GICv2 driver.


Julien Grall

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