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Re: [PATCH for-4.15] xen/vgic: Implement write to ISPENDR in vGICv{2, 3}

On 22/02/2021 13:45, Bertrand Marquis wrote:
Hi Julien,

On 20 Feb 2021, at 14:04, Julien Grall <julien@xxxxxxx> wrote:

From: Julien Grall <jgrall@xxxxxxxxxx>

Currently, Xen will send a data abort to a guest trying to write to the

Unfortunately, recent version of Linux (at least 5.9+) will start
writing to the register if the interrupt needs to be re-triggered
(see the callback irq_retrigger). This can happen when a driver (such as
the xgbe network driver on AMD Seattle) re-enable an interrupt:

(XEN) d0v0: vGICD: unhandled word write 0x00000004000000 to ISPENDR44
[   25.635837] Unhandled fault at 0xffff80001000522c
[   25.818716]  gic_retrigger+0x2c/0x38
[   25.822361]  irq_startup+0x78/0x138
[   25.825920]  __enable_irq+0x70/0x80
[   25.829478]  enable_irq+0x50/0xa0
[   25.832864]  xgbe_one_poll+0xc8/0xd8
[   25.836509]  net_rx_action+0x110/0x3a8
[   25.840328]  __do_softirq+0x124/0x288
[   25.844061]  irq_exit+0xe0/0xf0
[   25.847272]  __handle_domain_irq+0x68/0xc0
[   25.851442]  gic_handle_irq+0xa8/0xe0
[   25.855171]  el1_irq+0xb0/0x180
[   25.858383]  arch_cpu_idle+0x18/0x28
[   25.862028]  default_idle_call+0x24/0x5c
[   25.866021]  do_idle+0x204/0x278
[   25.869319]  cpu_startup_entry+0x24/0x68
[   25.873313]  rest_init+0xd4/0xe4
[   25.876611]  arch_call_rest_init+0x10/0x1c
[   25.880777]  start_kernel+0x5b8/0x5ec

As a consequence, the OS may become unusable.

Implementing the write part of ISPENDR is somewhat easy. For
virtual interrupt, we only need to inject the interrupt again.

For physical interrupt, we need to be more careful as the de-activation
of the virtual interrupt will be propagated to the physical distributor.
For simplicity, the physical interrupt will be set pending so the
workflow will not differ from a "real" interrupt.

Longer term, we could possible directly activate the physical interrupt
and avoid taking an exception to inject the interrupt to the domain.
(This is the approach taken by the new vGIC based on KVM).

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

This is something which will not be done by a guest very often so I think your
implementation actually makes it simpler and reduce possibilities of race 
so I am not even sure that the XXX comment is needed.

I think the XXX is useful as if someone notice an issue with the code, then they know what they could try.

I am open to suggestion how we could keep track of potential improvement.

But i am ok with it being in or not so:

Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>


Julien Grall



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