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    SubjectRe: [Qemu-devel] [PATCH v7] kvm: notify host when the guest is panicked
    Sasha Levin <> writes:

    > On 07/22/2012 09:22 PM, Anthony Liguori wrote:
    >> Sasha Levin <> writes:
    >>> On 07/21/2012 09:12 AM, Wen Congyang wrote:
    >>>> +#define KVM_PV_PORT (0x505UL)
    >>>> +
    >>>> #ifdef __KERNEL__
    >>>> #include <asm/processor.h>
    >>>> @@ -221,6 +223,11 @@ static inline void kvm_disable_steal_time(void)
    >>>> }
    >>>> #endif
    >>>> +static inline unsigned int kvm_arch_pv_features(void)
    >>>> +{
    >>>> + return inl(KVM_PV_PORT);
    >>>> +}
    >>>> +
    >>> Why is this safe?
    >>> I'm not sure you can just pick any ioport you'd like and use it.
    >> There are three ways I/O ports get used on a PC:
    >> 1) Platform devices
    >> - This is well defined since the vast majority of platform devices are
    >> implemented within a single chip. If you're emulating an i440fx
    >> chipset, the PIIX4 spec has an exhaustive list.
    >> 2) PCI devices
    >> - Typically, PCI only allocates ports starting at 0x0d00 to avoid
    >> conflicts with ISA devices.
    >> 3) ISA devices
    >> - ISA uses subtractive decoding so any ISA device can access. In
    >> theory, an ISA device could attempt to use port 0x0505 but it's
    >> unlikely. In a modern guest, there aren't really any ISA devices being
    >> added either.
    >> So yes, picking port 0x0505 is safe for something like this (as long as
    >> you check to make sure that you really are under KVM).
    > Is there anything that actually prevents me from using PCI ports lower
    > than 0x0d00? As you said in (3), ISA isn't really used anymore (nor is
    > implemented by lkvm for example), so placing PCI below 0x0d00 might
    > even make sense in that case.

    On modern systems, the OS goes by whatever is in the ACPI table
    describing the PCI bus. In QEMU, we have:

    WordIO (ResourceProducer, MinFixed, MaxFixed, PosDecode, EntireRange,
    0x0000, // Address Space Granularity
    0x0D00, // Address Range Minimum
    0xFFFF, // Address Range Maximum
    0x0000, // Address Translation Offset
    0xF300, // Address Length
    ,, , TypeStatic)

    So Linux will always use 0x0D00 -> 0xFFFF for the valid
    range. Practically speaking, you can't use anything below 0x0D00 because
    the PCI bus configuration registers live at 0xCF8-0xCFF. If you tried
    to create the region starting at 0x0500 you'd have to limit it to 0xCF8
    to avoid conflicting with the PCI host controller.

    That's not a useful amount of space for I/O ports so that would be a
    pretty dumb thing to do.

    > Furthermore, I can place one of these brand new virtio-mmio devices
    > which got introduced recently wherever I want right now - Having a
    > device that uses 0x505 would cause a pretty non-obvious failure mode.

    I think you're confusing PIO with MMIO. They are separate address

    You could certainly argue that relying on PIO is way too architecture
    specific since that's only available on x86. That's a good argument but
    the counter is that other architectures have their own interfaces for
    this sort of thing.

    > Either way, If we are going to grab an ioport, then:
    > - It should be documented well somewhere in Documentation/virt/kvm
    > - It should go through request_region() to actually claim those ioports.
    > - It should fail gracefully if that port is taken for some reason,
    > instead of not even checking it.

    I agree with the above.


    Anthony Liguori

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