Neo Jia wrote: > hi, > > I have to keep a 32-bit qmeu user space to work with some legacy > library I have but still want to use 64-bit host Linux to explore > 64-bit advantage. > > So I am wondering if I can use a 32-bit qemu + 64-bit kvm-kmod > configuration. Will there be any limitation or drawback for this > configuration? I already get one that we can't assign guest physical > memory more than 2047 MB. I use 32bit kvm on 64bit kernel since the day one. Nothing of interest since that, everything just works. Recently (this week) I come across a situation when something does not work in 64/32 mode. Namely it is linux aio (see the other thread in kvm@ a few days back) - but this is not due to kvm but due to other kernel subsystem (in this case aio) which lacks proper compat handlers in place. Generally I reported quite several issues in this config - here or there there were issues, something did not work. Now the places where we've issues are decreasing (hopefully anyway), at least I haven't seen issues recently, except of this aio stuff. But strictly speaking, I don't see any good reason to run 32bit kvm on 64 bit kernel either. Most distributions nowadays provide a set of 64bit libraries for their 32bit versions so that limited support for 64bit binaries are available. This is mostly enough for kvm - without X and SDL support it works just fine (using vnc display). Historically I've 32bit userspace, but most guests now are running with 64bit kvm - either because the guests switched to 64bit kernel or because aio thing or just because I looks like it is more efficient (less syscall/ioctl 32=>64 translation and the like). kvm itself uses only very few memory so here it almost makes no difference between 32 and 64 bits (in 64bit pointers are larger and hence usually more memory is used). Yes, it is difficult to provide everything needed for sdl, but for our tasks SDL windows aren't really necessary, and for testing 32bit mode works just fine too... /mjt -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
