At 07/23/2012 04:19 AM, Sasha Levin Wrote:
> On 07/22/2012 09:22 PM, Anthony Liguori wrote:
>> Sasha Levin <levinsasha...@gmail.com> 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.
>
> 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.
>
> 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.
Good idea.
> - It should fail gracefully if that port is taken for some reason, instead
> of not even checking it.
Yes, I agree it.
I will update it.
Thanks
Wen Congyang
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