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

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Message-ID: <41198c6c-22ad-1530-793a-e557f3c09415@oracle.com>
Date:   Mon, 18 Jul 2022 09:59:15 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.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 7/18/22 4:56 AM, Andrew Cooper wrote:
> 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".


The hypercall has been around for a while so I understand ABI concerns there but XEN_HVM_CPUID_UPCALL_VECTOR was introduced only a month ago. Why not tie presence of this bit to no longer having to explicitly set the callback field?


-boris