linux-kernel - Re: [RFC KERNEL PATCH v2 2/3] xen/pvh: Unmask irq for passthrough device in PVH dom0

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Message-ID: <b2a64f91-c202-4c28-aade-b38e2f8e47df@suse.com>
Date:   Wed, 13 Dec 2023 08:41:00 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     "Chen, Jiqian" <Jiqian.Chen@....com>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Huang, Ray" <Ray.Huang@....com>,
        Roger Pau Monné <roger.pau@...rix.com>
Subject: Re: [RFC KERNEL PATCH v2 2/3] xen/pvh: Unmask irq for passthrough
 device in PVH dom0

On 13.12.2023 08:14, Chen, Jiqian wrote:
> On 2023/12/12 19:39, Roger Pau Monné wrote:
>> On Tue, Dec 12, 2023 at 12:19:49PM +0100, Jan Beulich wrote:
>>> On 12.12.2023 12:18, Roger Pau Monné wrote:
>>>> On Tue, Dec 12, 2023 at 10:38:08AM +0100, Jan Beulich wrote:
>>>>> (I think the Cc list is too long here, but then I don't know who to
>>>>> keep and who to possibly drop.)
>>>>>
>>>>> On 12.12.2023 09:49, Roger Pau Monné wrote:
>>>>>> On Tue, Dec 12, 2023 at 06:16:43AM +0000, Chen, Jiqian wrote:
>>>>>>> On 2023/12/11 23:45, Roger Pau Monné wrote:
>>>>>>>> On Wed, Dec 06, 2023 at 06:07:26AM +0000, Chen, Jiqian wrote:
>>>>>>>>> +static int xen_pvh_setup_gsi(gsi_info_t *gsi_info)
>>>>>>>>> +{
>>>>>>>>> +       struct physdev_setup_gsi setup_gsi;
>>>>>>>>> +
>>>>>>>>> +       setup_gsi.gsi = gsi_info->gsi;
>>>>>>>>> +       setup_gsi.triggering = (gsi_info->trigger == ACPI_EDGE_SENSITIVE ? 0 : 1);
>>>>>>>>> +       setup_gsi.polarity = (gsi_info->polarity == ACPI_ACTIVE_HIGH ? 0 : 1);
>>>>>>>>> +
>>>>>>>>> +       return HYPERVISOR_physdev_op(PHYSDEVOP_setup_gsi, &setup_gsi);
>>>>>>>>> +}
>>>>>>>>
>>>>>>>> Hm, why not simply call pcibios_enable_device() from pciback?  What
>>>>>>> pcibios_enable_device had been called when using cmd "xl pci-assignable-add sbdf" from pciback. But it didn't do map_pirq and setup_gsi.
>>>>>>> Because pcibios_enable_device-> pcibios_enable_irq-> __acpi_register_gsi(acpi_register_gsi_ioapic PVH specific)
>>>>>>>> you are doing here using the hypercalls is a backdoor into what's done
>>>>>>>> automatically by Xen on IO-APIC accesses by a PVH dom0.
>>>>>>> But the gsi didn't be unmasked, and vioapic_hwdom_map_gsi is never called.
>>>>>>> So, I think in pciback, if we can do what vioapic_hwdom_map_gsi does.
>>>>>>>
>>>>>>
>>>>>> I see, it does setup the IO-APIC pin but doesn't unmask it, that's
>>>>>> what I feared.
>>>>>>
>>>>>>>> It will be much more natural for the PVH dom0 model to simply use the
>>>>>>>> native way to configure and unmask the IO-APIC pin, and that would
>>>>>>>> correctly setup the triggering/polarity and bind it to dom0 without
>>>>>>>> requiring the usage of any hypercalls.
>>>>>>> Do you still prefer that I called unmask_irq in pcistub_init_device, as this v2 patch do?
>>>>>>> But Thomas Gleixner think it is not suitable to export unmask_irq.
>>>>>>
>>>>>> Yeah, that wasn't good.
>>>>>>
>>>>>>>>
>>>>>>>> Is that an issue since in that case the gsi will get mapped and bound
>>>>>>>> to dom0?
>>>>>>> Dom0 do map_pirq is to pass the check xc_domain_irq_permission()-> pirq_access_permitted(), 
>>>>>>
>>>>>> Can we see about finding another way to fix this check?
>>>>>>
>>>>>> One option would be granting permissions over the IRQ in
>>>>>> PHYSDEVOP_setup_gsi?
>>>>>
>>>>> There's no domain available there, and imo it's also the wrong interface to
>>>>> possibly grant any permissions.
>>>>
>>>> Well, the domain is the caller.
>>>
>>> Granting permission to itself?
>>
>> See below in the previous email, the issue is not with the
>> permissions, which are correctly assigned from
>> dom0_setup_permissions(), but the usage of domain_pirq_to_irq() in
>> pirq_access_permitted() as called by XEN_DOMCTL_irq_permission.
>> There's no need to play with the permissions at all.
> Yes, the problem is pci_add_dm_done-> xc_domain_irq_permission-> XEN_DOMCTL_irq_permission-> pirq_access_permitted->domain_pirq_to_irq->return irq is 0, so it failed.
> I am think that since the PVH doesn't use pirq, can we just skip this irq_permission check for PVH?

If not the pIRQ, then the real IRQ would need checking for permissions.
You won't get away without any checking at all, I'm afraid.

Jan