32-bit qemu + 64-bit kvm be a problem?

32-bit qemu + 64-bit kvm be a problem?

[Neo Jia]

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.

Thanks,
Neo
-- 
I would remember that if researchers were not ambitious
probably today we haven't the technology we are using!

Re: 32-bit qemu + 64-bit kvm be a problem?

[Avi Kivity]

On 03/10/2010 11:59 AM, 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.

It's fully supported.  It's less well tested that 64/64 or 32/32, so 
please report any bugs.

> 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.
>    

That is the only limitation AFAIK.

-- 
error compiling committee.c: too many arguments to function

Re: 32-bit qemu + 64-bit kvm be a problem?

[Michael Tokarev]

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

Re: 32-bit qemu + 64-bit kvm be a problem?

[Neo Jia]

On Wed, Mar 10, 2010 at 2:12 PM, Michael Tokarev <mjt@tls.msk.ru> wrote:
> 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.

Michael,

I just came back to this thread because I am seeing that I can't run
VISTA 64-bit inside 64/32 mode, which will crash with bugcheck 0x5D.
Is this a known issue?

>
> 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.

Could you tell me more about the AIO issue? Will this will slow down
the guest if it does a lot I/O? Will setting up coalescing help?

Thanks,
Neo


>
> 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
>



-- 
I would remember that if researchers were not ambitious
probably today we haven't the technology we are using!

Re: 32-bit qemu + 64-bit kvm be a problem?

[Michael Tokarev]

02.06.2010 09:44, Neo Jia wrote:
> On Wed, Mar 10, 2010 at 2:12 PM, Michael Tokarev<mjt@tls.msk.ru>  wrote:
[]
>> I use 32bit kvm on 64bit kernel since the day one.  Nothing of interest
>> since that, everything just works.

> I just came back to this thread because I am seeing that I can't run
> VISTA 64-bit inside 64/32 mode, which will crash with bugcheck 0x5D.
> Is this a known issue?

I haven't seen it myself, but I never tried vista.  Win7 64bit installs
and works just fine.  STOP 0x0000005D, according to M$, means "there is
a processor present that is not supported".  I'd say try to experiment
with different -cpu options.  Also see if it works in native 64bit mode.

>> 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.
>
> Could you tell me more about the AIO issue? Will this will slow down
> the guest if it does a lot I/O? Will setting up coalescing help?

You can search it yourself.  In a nutshell it's an optimisation in kvm
since version 0.12  to let raw devices be used a bit more efficient.

/mjt