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Re: [Qemu-devel] [PATCH v7] kvm: notify host when the guest is panicked

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v7] kvm: notify host when the guest is panicked
Date: Sun, 22 Jul 2012 17:20:48 -0500
User-agent: Notmuch/0.13.2+60~g7ecf77d (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu)

Sasha Levin <address@hidden> writes:

> On 07/22/2012 09:22 PM, Anthony Liguori wrote:
>> Sasha Levin <address@hidden> 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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