Re: [PATCH V3 2/3] HYPERV/IOMMU: Add Hyper-V stub IOMMU driver

[Tianyu Lan]

Hi Alex:
            Thanks for your review.

On Fri, Feb 8, 2019 at 2:15 AM Alex Williamson
<alex.williamson@xxxxxxxxxx> wrote:
>
> On Thu,  7 Feb 2019 23:33:48 +0800
> lantianyu1986@xxxxxxxxx wrote:
>
> > From: Lan Tianyu <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > On the bare metal, enabling X2APIC mode requires interrupt remapping
> > function which helps to deliver irq to cpu with 32-bit APIC ID.
> > Hyper-V doesn't provide interrupt remapping function so far and Hyper-V
> > MSI protocol already supports to deliver interrupt to the CPU whose
> > virtual processor index is more than 255. IO-APIC interrupt still has
> > 8-bit APIC ID limitation.
> >
> > This patch is to add Hyper-V stub IOMMU driver in order to enable
> > X2APIC mode successfully in Hyper-V Linux guest. The driver returns X2APIC
> > interrupt remapping capability when X2APIC mode is available. Otherwise,
> > it creates a Hyper-V irq domain to limit IO-APIC interrupts' affinity
> > and make sure cpus assigned with IO-APIC interrupt have 8-bit APIC ID.
> >
> > Define 24 IO-APIC remapping entries because Hyper-V only expose one
> > single IO-APIC and one IO-APIC has 24 pins according IO-APIC spec(
> > https://pdos.csail.mit.edu/6.828/2016/readings/ia32/ioapic.pdf).
> >
> > Signed-off-by: Lan Tianyu <Tianyu.Lan@xxxxxxxxxxxxx>
> > ---
> > Change since v2:
> >        - Improve comment about why save IO-APIC entry in the irq chip data.
> >        - Some code improvement.
> >        - Improve statement in the IOMMU Kconfig.
> >
> > Change since v1:
> >       - Remove unused pr_fmt
> >       - Make ioapic_ir_domain as static variable
> >       - Remove unused variables cfg and entry in the hyperv_irq_remapping_alloc()
> >       - Fix comments
> > ---
> >  drivers/iommu/Kconfig         |   8 ++
> >  drivers/iommu/Makefile        |   1 +
> >  drivers/iommu/hyperv-iommu.c  | 194 ++++++++++++++++++++++++++++++++++++++++++
> >  drivers/iommu/irq_remapping.c |   3 +
> >  drivers/iommu/irq_remapping.h |   1 +
> >  5 files changed, 207 insertions(+)
> >  create mode 100644 drivers/iommu/hyperv-iommu.c
> ...
> > diff --git a/drivers/iommu/hyperv-iommu.c b/drivers/iommu/hyperv-iommu.c
> > new file mode 100644
> > index 0000000..d8572c5
> > --- /dev/null
> > +++ b/drivers/iommu/hyperv-iommu.c
> ...
> > +static int __init hyperv_prepare_irq_remapping(void)
> > +{
> > +     struct fwnode_handle *fn;
> > +     int i;
> > +
> > +     if (!hypervisor_is_type(x86_hyper_type) ||
> > +         !x2apic_supported())
> > +             return -ENODEV;
> > +
> > +     fn = irq_domain_alloc_named_id_fwnode("HYPERV-IR", 0);
> > +     if (!fn)
> > +             return -ENOMEM;
> > +
> > +     ioapic_ir_domain =
> > +             irq_domain_create_hierarchy(arch_get_ir_parent_domain(),
> > +                             0, IOAPIC_REMAPPING_ENTRY, fn,
> > +                             &hyperv_ir_domain_ops, NULL);
> > +
> > +     irq_domain_free_fwnode(fn);
> > +
> > +     /*
> > +      * Hyper-V doesn't provide irq remapping function for
> > +      * IO-APIC and so IO-APIC only accepts 8-bit APIC ID.
> > +      * Cpu's APIC ID is read from ACPI MADT table and APIC IDs
> > +      * in the MADT table on Hyper-v are sorted monotonic increasingly.
> > +      * APIC ID reflects cpu topology. There maybe some APIC ID
> > +      * gaps when cpu number in a socket is not power of two. Prepare
> > +      * max cpu affinity for IOAPIC irqs. Scan cpu 0-255 and set cpu
> > +      * into ioapic_max_cpumask if its APIC ID is less than 256.
> > +      */
> > +     for (i = 0; i < 256; i++)
> > +             if (cpu_physical_id(i) < 256)
> > +                     cpumask_set_cpu(i, &ioapic_max_cpumask);
>
> This looks sketchy.  What if NR_CPUS is less than 256?  Thanks,

Nice catch. I should check NR_CPUS here. Will update. Thanks.


-- 
Best regards
Tianyu Lan