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

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Date:   Mon, 25 Jul 2022 16:46:53 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jane Malalane <Jane.Malalane@...rix.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Xen-devel <xen-devel@...ts.xenproject.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>
Subject: Re: [PATCH] x86/xen: Add support for HVMOP_set_evtchn_upcall_vector


On 7/25/22 6:03 AM, Jane Malalane wrote:
> On 18/07/2022 14:59, Boris Ostrovsky wrote:
>> 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?
>>
> Any other opinions on this?
>
> (i.e., calling xen_set_callback_via(1) after
> HVMOP_set_evtchn_upcall_vector OR not exposing this to guests and
> instead having Xen call this function (in hvmop_set_evtchn_upcall_vector
> maybe) and tieing its presense to XEN_HVM_CPUID_UPCALL_VECTOR which was
> recently added)


CPUID won't help here, I wasn't thinking clearly.


Can we wrap the HVMOP_set_evtchn_upcall_vector hypercall in a function that will decide whether or not to also do xen_set_callback_via(1)?


-boris