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

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Date:   Mon, 18 Jul 2022 08:56:02 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jane Malalane <Jane.Malalane@...rix.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 14:10, Boris Ostrovsky wrote:
>
> On 7/15/22 5:50 AM, Andrew Cooper wrote:
>> 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.
>
>
> Feels like it (setting the callback parameter) is something that the
> hypervisor should do --- no need to expose guests to this.

Sensible or not, it is the ABI.

Linux still needs to work (nicely) with older Xen's in the world, and we
can't just retrofit a change in the hypervisor which says "btw, this ABI
we've just changed now has a side effect of modifying a field that you
also logically own".

~Andrew