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Re: [PATCH] x86/HVM: don't mark evtchn upcall vector as pending when vLAPIC is disabled
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 24 Nov 2022 10:11:05 +0100
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- Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Paul Durrant <paul@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 24 Nov 2022 09:11:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24.11.2022 10:06, Roger Pau Monné wrote:
> On Thu, Nov 24, 2022 at 09:42:40AM +0100, Roger Pau Monné wrote:
>> On Thu, Nov 24, 2022 at 08:59:00AM +0100, Jan Beulich wrote:
>>> - problematic wrt evtchn_upcall_pending, once set, preventing event
>>> injection later on.
>>> As you may have inferred already, I'm inclined to suggest to drop the
>>> the is_vcpu_online() check from hvm_set_callback_via().
>>>
>>> One related question here is whether vlapic_do_init() shouldn't have
>>> the non-architectural side effect of clearing evtchn_upcall_pending.
>>> While this again violates the principle of the hypervisor only ever
>>> setting that bit, it would deal with the risk of no further event
>>> injection once the flag is set, considering that vlapic_do_init()
>>> clears IRR (and ISR).
>>
>> That would seem sensible to me, and was kind of what I was suggesting
>> in:
>>
>> https://lore.kernel.org/xen-devel/Y3eO0bMKRPYJc2yQ@Air-de-Roger/
>
> Another option would be for vcpu_mark_events_pending() to
> unconditionally call hvm_assert_evtchn_irq() regardless of the state
> of evtchn_upcall_pending.
I think you said so before, and ...
> This will create some spurious events.
... I continue to be afraid of s/some/many/. This event delivery is meant
to be kind of edge triggered, and I think it is a reasonable model to treat
the flag going from 0 to 1 as the "edge".
Jan
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