linux-kernel - Re: [PATCH] x86/xen: Add support for HVMOP_set_evtchn_upcall

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Message-ID: <01cc9e39-242a-3cbf-13b3-cb3ecccd5737@citrix.com>
Date:   Fri, 15 Jul 2022 09:50:11 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Jane Malalane <Jane.Malalane@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
CC:     Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Maximilian Heyne <mheyne@...zon.de>,
        Jan Beulich <jbeulich@...e.com>,
        Colin Ian King <colin.king@...el.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] x86/xen: Add support for HVMOP_set_evtchn_upcall_vector

On 15/07/2022 09:18, Jane Malalane wrote:
> On 14/07/2022 00:27, Boris Ostrovsky wrote:
>>>       xen_hvm_smp_init();
>>>       WARN_ON(xen_cpuhp_setup(xen_cpu_up_prepare_hvm, xen_cpu_dead_hvm));
>>> diff --git a/arch/x86/xen/suspend_hvm.c b/arch/x86/xen/suspend_hvm.c
>>> index 9d548b0c772f..be66e027ef28 100644
>>> --- a/arch/x86/xen/suspend_hvm.c
>>> +++ b/arch/x86/xen/suspend_hvm.c
>>> @@ -5,6 +5,7 @@
>>>   #include <xen/hvm.h>
>>>   #include <xen/features.h>
>>>   #include <xen/interface/features.h>
>>> +#include <xen/events.h>
>>>   #include "xen-ops.h"
>>> @@ -14,6 +15,23 @@ void xen_hvm_post_suspend(int suspend_cancelled)
>>>           xen_hvm_init_shared_info();
>>>           xen_vcpu_restore();
>>>       }
>>> -    xen_setup_callback_vector();
>>> +    if (xen_ack_upcall) {
>>> +        unsigned int cpu;
>>> +
>>> +        for_each_online_cpu(cpu) {
>>> +            xen_hvm_evtchn_upcall_vector_t op = {
>>> +                    .vector = HYPERVISOR_CALLBACK_VECTOR,
>>> +                    .vcpu = per_cpu(xen_vcpu_id, cpu),
>>> +            };
>>> +
>>> +            BUG_ON(HYPERVISOR_hvm_op(HVMOP_set_evtchn_upcall_vector,
>>> +                         &op));
>>> +            /* Trick toolstack to think we are enlightened. */
>>> +            if (!cpu)
>>> +                BUG_ON(xen_set_callback_via(1));
>>
>> What are you trying to make the toolstack aware of? That we have *a* 
>> callback (either global or percpu)?
> Yes, specifically for the check in libxl__domain_pvcontrol_available.

And others.

This is all a giant bodge, but basically a lot of tooling uses the
non-zero-ness of the CALLBACK_VIA param to determine whether the VM has
Xen-aware drivers loaded or not.

The value 1 is a CALLBACK_VIA value which encodes GSI 1, and the only
reason this doesn't explode everywhere is because the
evtchn_upcall_vector registration takes priority over GSI delivery.

This is decades of tech debt piled on top of tech debt.

~Andrew