Re: [Qemu-devel] [PATCH 15/35] tcg-s390: Query instruction extensions that are installed.

[Alexander Graf]

On 13.06.2010, at 18:44, Aurelien Jarno wrote:

> On Sun, Jun 13, 2010 at 09:02:40AM -0700, Richard Henderson wrote:
>> On 06/13/2010 03:49 AM, Aurelien Jarno wrote:
>>>> Also, what era is that second machine without highgprs?  Is it running an
>>>> old kernel, or a 32-bit kernel?
>>> 
>>> I have very few infos about it, it's an IBM System z10 machine running a
>>> 64-bit 2.6.26 kernel.
>> 
>> Ah, I see it now: ea2a4d3a3a929ef494952bba57a0ef1a8a877881
>> 
>>    [S390] 64-bit register support for 31-bit processes
>> 
>> which adds a mechanism to pass the high parts of the gprs
>> in the ucontext to the 31-bit signal handler, and adds a
>> spot for them in the 31-bit core dump.
>> 
>> It doesn't change the actual saving of registers within
>> the kernel.  Since we take asynchronous signals and return
>> from them (as opposed to always longjmping out), we cannot
>> use the full 64-bit register within a 31-bit process without
>> having that bit set in HWCAP.
>> 
>> Something to remember if we ever implement TCG for 31-bit mode.
>> At the moment we only allow KVM in 31-bit mode.
>> 
> 
> Is KVM in 31-bit mode actually functional?

I'm not aware of anything preventing it to be. But I honestly haven't tried. As 
long as all hypercall parameters stay within the first 32/31 bits, things 
should be safe.

Alex