On 07/22/2012 10:19 PM, 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. > - 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.