Re: [PATCH v2 1/2] x86/vpt: execute callbacks for masked interrupts

From: "Jan Beulich" <JBeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	Stefan Bader <stefan.bader@canonical.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 1/2] x86/vpt: execute callbacks for masked interrupts
Date: Tue, 10 Apr 2018 03:33:53 -0600	[thread overview]
Message-ID: <5ACCA12102000078001B9DBE@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <20180410085304.xvdwq54dpixfwau6@MacBook-Pro-de-Roger.local>

>>> On 10.04.18 at 10:53, <roger.pau@citrix.com> wrote:
> On Mon, Apr 09, 2018 at 09:34:57AM -0600, Jan Beulich wrote:
>> >>> On 30.03.18 at 14:35, <roger.pau@citrix.com> wrote:
>> > Execute periodic_time callbacks even if the interrupt is not actually
>> > injected because the IRQ is masked.
>> > 
>> > Current callbacks from emulated timer devices only update emulated
>> > registers, which from my reading of the specs should happen regardless
>> > of whether the interrupt has been injected or not.
>> 
>> While generally I agree, it also means extra work done. Looking
>> at the PIT case, for example, there's no strict need to do the
>> update when the IRQ is masked, as the value being updated is
>> only used to subtract from get_guest_time()'s return value.
>> Similarly for the LAPIC case.
>> 
>> In the RTC case your change actually looks risky, due to the
>> pt_dead_ticks logic. I can't help getting the impression that the
>> IRQ being off for 10 ticks would lead to no RTC interrupts at all
>> anymore for the guest (until something resets that counter),
>> which seems wrong to me.
> 
> Hm, right. The RTC is already handled specially in order to not
> disable the timer but also don't call the handler if the IRQ is
> masked.
> 
> Maybe the right solution is to add some flags to the vpt code,
> something like:
> 
>  - DISABLE_ON_MASKED: only valid for periodic interrupts. Destroy the
>    timer if the IRQ is masked when the timer fires.
>  - SKIP_CALLBACK_ON_MASKED: do not execute the timer callback if the
>    IRQ is masked when the timer fires.
> 
> That AFAICT should allow Xen to keep the previous behaviour for
> existing timer code (and remove the RTC special casing).

Something like this, yes (I don't really like the names you suggest,
but I also can't suggest any better ones right away).

Jan

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