linux-kernel - Re: [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd

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Message-ID: <05661003-64f0-a32a-5659-6463d4806ef9@oracle.com>
Date:   Mon, 30 Nov 2020 17:15:31 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Ankur Arora <ankur.a.arora@...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 11/39] KVM: x86/xen: evtchn signaling via eventfd

On 11/30/20 4:48 PM, David Woodhouse wrote:
> On Mon, 2020-11-30 at 15:08 +0000, Joao Martins wrote:
>> On 11/30/20 12:55 PM, David Woodhouse wrote:
>>> On Mon, 2020-11-30 at 12:17 +0000, Joao Martins wrote:
>>>> On 11/30/20 9:41 AM, David Woodhouse wrote:
>>>>> On Wed, 2019-02-20 at 20:15 +0000, Joao Martins wrote:
>>>> One thing I didn't quite do at the time, is the whitelisting of unregistered
>>>> ports to userspace. Right now, it's a blacklist i.e. if it's not handled in
>>>> the kernel (IPIs, timer vIRQ, etc) it goes back to userspace. When the only
>>>> ones which go to userspace should be explicitly requested as such
>>>> and otherwise return -ENOENT in the hypercall.
>>>
>>> Hm, why would -ENOENT be a fast path which needs to be handled in the
>>> kernel?
>>>
>>
>> It's not that it's a fast path.
>>
>> Like sending an event channel to an unbound vector, now becomes an possible vector to
>> worry about in userspace VMM e.g. should that port lookup logic be fragile.
>>
>> So it's more along the lines of Nack-ing the invalid port earlier to rather go
>> to go userspace to invalidate it, provided we do the lookup anyway in the kernel.
> 
> If the port lookup logic is fragile, I *want* it in the sandboxed
> userspace VMM and not in the kernel :)
> 
Yes definitely -- I think we are on the same page on that.

But it's just that we do the lookup *anyways* to check if the kernel has a given
evtchn port registered. That's the lookup I am talking about here, with just an
extra bit to tell that's a userspace handled port.

> And unless we're going to do *all* of the EVTCHNOP_bind*, EVTCHN_close,
> etc. handling in the kernel, doesn't userspace have to have all that
> logic for managing the port space anyway?
> 
Indeed.

> I think it's better to let userspace own it outright, and use the
> kernel bypass purely for the fast paths. The VMM can even implement
> IPI/VIRQ support in userspace, then use the kernel bypass if/when it's
> available.
> 
True, and it's pretty much how it's implemented today.

But felt it was still worth having this discussion ... should this be
considered or discarded. I suppose we stick with the later for now.

>>>> Perhaps eventfd could be a way to express this? Like if you register
>>>> without an eventfd it's offloaded, otherwise it's assigned to userspace,
>>>> or if neither it's then returned an error without bothering the VMM.
>>>
>>> I much prefer the simple model where the *only* event channels that the
>>> kernel knows about are the ones it's expected to handle.
>>>
>>> For any others, the bypass doesn't kick in, and userspace gets the
>>> KVM_EXIT_HYPERCALL exit.
>>>
>>
>> /me nods
>>
>> I should comment on your other patch but: if we're going to make it generic for
>> the userspace hypercall handling, might as well move hyper-v there too. In this series,
>> I added KVM_EXIT_XEN, much like it exists KVM_EXIT_HYPERV -- but with a generic version
>> I wonder if a capability could gate KVM_EXIT_HYPERCALL to handle both guest types, while
>> disabling KVM_EXIT_HYPERV. But this is probably subject of its own separate patch :)
> 
> There's a limit to how much consolidation we can do because the ABI is
> different; the args are in different registers.
> 
Yes. It would be optionally enabled of course and VMM would have to adjust to the new ABI
-- surely wouldn't want to break current users of KVM_EXIT_HYPERV.

> I do suspect Hyper-V should have marshalled its arguments into the
> existing kvm_run->arch.hypercall and used KVM_EXIT_HYPERCALL but I
> don't think it makes sense to change it now since it's a user-facing
> ABI. I don't want to follow its lead by inventing *another* gratuitous
> exit type for Xen though.
> 
I definitely like the KVM_EXIT_HYPERCALL better than a KVM_EXIT_XEN userspace
exit type ;)

But I guess you still need to co-relate a type of hypercall (Xen guest cap enabled?) to
tell it's Xen or KVM to specially enlighten certain opcodes (EVTCHNOP_send).

	Joao