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

[Sasha Levin]

On 07/22/2012 10:19 PM, Sasha Levin wrote:
> 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.
> 
> 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.
>  - It should fail gracefully if that port is taken for some reason, instead 
> of not even checking it.
> 

Out of curiosity I tested that, and apparently lkvm has no problem allocating 
virtio-pci devices in that range:

sh-4.2# pwd
/sys/devices/pci0000:00/0000:00:01.0
sh-4.2# cat resource | head -n1
0x0000000000000500 0x00000000000005ff 0x0000000000040101

This was with the commit in question applied.