AMD-Intel guest migration and CPUs without NX

AMD-Intel guest migration and CPUs without NX

[Tomasz Chmielewski]

I have an older Intel CPU which doesn't support NX (/proc/cpuinfo - 
below). Is it safe to migrate guests running on newer CPUs to this older 
CPU? I made some simple tests and migration works, but I'm not sure if 
the guests will be stable after such migration.


I'm also a bit confused a bit over what the documentation says:

According to http://www.linux-kvm.org/page/Migration:

   There are some older Intel processors which don't support NX (or XD),
   which may cause problems in a cluster which includes NX-supporting
   hosts. We may add a feature to hide NX if this proves to be a problem
   in actual deployments.


So the above says I may have some problems.

On the other hand, FAQ below seems to indicate that migration on 64 bit 
hosts should be fine (even when they don't support NX?), only 32 bit 
hosts may have problems:

http://www.linux-kvm.org/page/FAQ

   Yes. There may be issues on 32-bit Intel hosts which don't support NX
   (or XD), but for 64-bit hosts back and forth migration should work
   well. Migration of 32-bit guests should work between 32-bit hosts and
   64-bit hosts.



processor       : 1
vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Intel(R) Xeon(R) CPU            3050  @ 2.13GHz
stepping        : 6
cpu MHz         : 2133.410
cache size      : 2048 KB
physical id     : 0
siblings        : 2
core id         : 1
cpu cores       : 2
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge 
mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe 
syscall lm constant_tsc arch_perfmon pebs bts rep_good pni monitor 
ds_cpl vmx est tm2 ssse3 cx16 xtpr lahf_lm
bogomips        : 4266.87
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:


-- 
Tomasz Chmielewski
http://wpkg.org

Re: AMD-Intel guest migration and CPUs without NX

[Avi Kivity]

Tomasz Chmielewski wrote:
> I have an older Intel CPU which doesn't support NX (/proc/cpuinfo - 
> below). Is it safe to migrate guests running on newer CPUs to this 
> older CPU? I made some simple tests and migration works, but I'm not 
> sure if the guests will be stable after such migration.
>
>
> I'm also a bit confused a bit over what the documentation says:
>
> According to http://www.linux-kvm.org/page/Migration:
>
>   There are some older Intel processors which don't support NX (or XD),
>   which may cause problems in a cluster which includes NX-supporting
>   hosts. We may add a feature to hide NX if this proves to be a problem
>   in actual deployments.
>
>
> So the above says I may have some problems.

Right, it's not safe in general.  It may work if the guest doesn't use 
NX.  It may also work if the guest does not rely on NX working properly.

>
> On the other hand, FAQ below seems to indicate that migration on 64 
> bit hosts should be fine (even when they don't support NX?), only 32 
> bit hosts may have problems:
>
> http://www.linux-kvm.org/page/FAQ
>
>   Yes. There may be issues on 32-bit Intel hosts which don't support NX
>   (or XD), but for 64-bit hosts back and forth migration should work
>   well. Migration of 32-bit guests should work between 32-bit hosts and
>   64-bit hosts.

Looks like that paragraph assumes that all 64-bit hosts have NX.  Your 
/proc/cpuinfo proves otherwise.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

Re: AMD-Intel guest migration and CPUs without NX

[Tomasz Chmielewski]

Avi Kivity schrieb:

>> So the above says I may have some problems.
> 
> Right, it's not safe in general.  It may work if the guest doesn't use 
> NX.  It may also work if the guest does not rely on NX working properly.

Although I think my guests don't use it, you never know.
Is it possible to disable NX for chosen guests?


>> On the other hand, FAQ below seems to indicate that migration on 64 
>> bit hosts should be fine (even when they don't support NX?), only 32 
>> bit hosts may have problems:
>>
>> http://www.linux-kvm.org/page/FAQ
>>
>>   Yes. There may be issues on 32-bit Intel hosts which don't support NX
>>   (or XD), but for 64-bit hosts back and forth migration should work
>>   well. Migration of 32-bit guests should work between 32-bit hosts and
>>   64-bit hosts.
> 
> Looks like that paragraph assumes that all 64-bit hosts have NX.  Your 
> /proc/cpuinfo proves otherwise.

-- 
Tomasz Chmielewski
http://wpkg.org

Re: AMD-Intel guest migration and CPUs without NX

[Avi Kivity]

Tomasz Chmielewski wrote:
> Although I think my guests don't use it, you never know.
> Is it possible to disable NX for chosen guests?

-cpu qemu64,-nx

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

Re: AMD-Intel guest migration and CPUs without NX

[Tomasz Chmielewski]

Avi Kivity schrieb:
> Tomasz Chmielewski wrote:
>> Although I think my guests don't use it, you never know.
>> Is it possible to disable NX for chosen guests?
> 
> -cpu qemu64,-nx

Thanks.
I updated the FAQ and migration pages to contain this information.



-- 
Tomasz Chmielewski
http://wpkg.org