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Date:   Wed, 2 Dec 2020 17:08:06 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Joao Martins <joao.m.martins@...cle.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 10/39] KVM: x86/xen: support upcall vector

On 2020-12-02 11:02 a.m., David Woodhouse wrote:
> On Wed, 2020-12-02 at 18:34 +0000, Joao Martins wrote:
>> On 12/2/20 4:47 PM, David Woodhouse wrote:
>>> On Wed, 2020-12-02 at 13:12 +0000, Joao Martins wrote:
>>>> On 12/2/20 11:17 AM, David Woodhouse wrote:
>>>>> I might be more inclined to go for a model where the kernel handles the
>>>>> evtchn_pending/evtchn_mask for us. What would go into the irq routing
>>>>> table is { vcpu, port# } which get passed to kvm_xen_evtchn_send().
>>>>
>>>> But passing port to the routing and handling the sending of events wouldn't it lead to
>>>> unnecessary handling of event channels which aren't handled by the kernel, compared to
>>>> just injecting caring about the upcall?
>>>
>>> Well, I'm generally in favour of *not* doing things in the kernel that
>>> don't need to be there.
>>>
>>> But if the kernel is going to short-circuit the IPIs and VIRQs, then
>>> it's already going to have to handle the evtchn_pending/evtchn_mask
>>> bitmaps, and actually injecting interrupts.
>>>
>>
>> Right. I was trying to point that out in the discussion we had
>> in next patch. But true be told, more about touting the idea of kernel
>> knowing if a given event channel is registered for userspace handling,
>> rather than fully handling the event channel.
>>
>> I suppose we are able to provide both options to the VMM anyway
>> i.e. 1) letting them handle it enterily in userspace by intercepting
>> EVTCHNOP_send, or through the irq route if we want kernel to offload it.
> 
> Right. The kernel takes what it knows about and anything else goes up
> to userspace.
> 
> I do like the way you've handled the vcpu binding in userspace, and the
> kernel just knows that a given port goes to a given target CPU.
> 
>>
>>> For the VMM
>>> API I think we should follow the Xen model, mixing the domain-wide and
>>> per-vCPU configuration. It's the best way to faithfully model the
>>> behaviour a true Xen guest would experience.
>>>
>>> So KVM_XEN_ATTR_TYPE_CALLBACK_VIA can be used to set one of
>>>   • HVMIRQ_callback_vector, taking a vector#
>>>   • HVMIRQ_callback_gsi for the in-kernel irqchip, taking a GSI#
>>>
>>> And *maybe* in a later patch it could also handle
>>>   • HVMIRQ_callback_gsi for split-irqchip, taking an eventfd
>>>   • HVMIRQ_callback_pci_intx, taking an eventfd (or a pair, for EOI?)
>>>
>>
>> Most of the Xen versions we were caring had callback_vector and
>> vcpu callback vector (despite Linux not using the latter). But if you're
>> dating back to 3.2 and 4.1 well (or certain Windows drivers), I suppose
>> gsi and pci-intx are must-haves.
> 
> Note sure about GSI but PCI-INTX is definitely something I've seen in
> active use by customers recently. I think SLES10 will use that.
> 
>> I feel we could just accommodate it as subtype in KVM_XEN_ATTR_TYPE_CALLBACK_VIA.
>> Don't see the adavantage in having another xen attr type.
> 
> Yeah, fair enough.
> 
>> But kinda have mixed feelings in having kernel handling all event channels ABI,
>> as opposed to only the ones userspace asked to offload. It looks a tad unncessary besides
>> the added gain to VMMs that don't need to care about how the internals of event channels.
>> But performance-wise it wouldn't bring anything better. But maybe, the former is reason
>> enough to consider it.
> 
> Yeah, we'll see. Especially when it comes to implementing FIFO event
> channels, I'd rather just do it in one place — and if the kernel does
> it anyway then it's hardly difficult to hook into that.

Sorry I'm late to this conversation. Not a whole lot to add to what Joao
said. I would only differ with him on how much to offload.

Given that we need the fast path in the kernel anyway, I think it's simpler
to do all the event-channel bitmap only in the kernel.
This would also simplify using the kernel Xen drivers if someone eventually
decides to use them.


Ankur

> 
> But I've been about as coherent as I can be in email, and I think we're
> generally aligned on the direction. I'll do some more experiments and
> see what I can get working, and what it looks like.
> 
> I'm focusing on making the shinfo stuff all use kvm_map_gfn() first.
> 

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