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d981059e13
When a Linux VM runs on Hyper-V, if the VM has CPUs with >255 APIC IDs, the CPUs can't be the destination of IOAPIC interrupts, because the IOAPIC RTE's Dest Field has only 8 bits. Currently the hackery driver drivers/iommu/hyperv-iommu.c is used to ensure IOAPIC interrupts are only routed to CPUs that don't have >255 APIC IDs. However, there is an issue with kdump, because the kdump kernel can run on any CPU, and hence IOAPIC interrupts can't work if the kdump kernel run on a CPU with a >255 APIC ID. The kdump issue can be fixed by the Extended Dest ID, which is introduced recently by David Woodhouse (for IOAPIC, see the field virt_destid_8_14 in struct IO_APIC_route_entry). Of course, the Extended Dest ID needs the support of the underlying hypervisor. The latest Hyper-V has added the support recently: with this commit, on such a Hyper-V host, Linux VM does not use hyperv-iommu.c because hyperv_prepare_irq_remapping() returns -ENODEV; instead, Linux kernel's generic support of Extended Dest ID from David is used, meaning that Linux VM is able to support up to 32K CPUs, and IOAPIC interrupts can be routed to all the CPUs. On an old Hyper-V host that doesn't support the Extended Dest ID, nothing changes with this commit: Linux VM is still able to bring up the CPUs with > 255 APIC IDs with the help of hyperv-iommu.c, but IOAPIC interrupts still can not go to such CPUs, and the kdump kernel still can not work properly on such CPUs. [ tglx: Updated comment as suggested by David ] Signed-off-by: Dexuan Cui <decui@microsoft.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: David Woodhouse <dwmw@amazon.co.uk> Link: https://lore.kernel.org/r/20201103011136.59108-1-decui@microsoft.com |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.