On 28.08.2017 16:24, Christian Borntraeger wrote: > > > On 08/28/2017 04:06 PM, David Hildenbrand wrote: >> On 28.08.2017 10:07, Christian Borntraeger wrote: >>> From: "Jason J. Herne" <jjherne@xxxxxxxxxxxxxxxxxx> >>> >>> kvm has always supported the concept of starting in z/Arch mode so let's >>> reflect the feature bit to the guest. >>> >>> Also, we change sigp set architecture to reject any request to change >>> architecture modes. >> >> Hm ... this seems to imply that CZAM is always set, but what about >> running on old user space (possibly on old hw)? Old QEMU will not enable >> CZAM. > > 3 cases. > 1. very old QEMU without user sigp > 2. old QEMU with user sigp/without CPU model > 3. new QEMU with user sigp/cpu model > > I think we agree that cases 2 and 3 should not matter at all for this kernel patch > as the sigp is handled by QEMU. > > > > This is case 1: >> And especially old user space will rely on SET ARCHITECTURE being >> handled in the kernel. > > > Yes, and it continues to be handled in the kernel. It is just that the guest > will now see a different sigp return code. Before, our sigp implementation lied > to the guest in a way that worked for Linux (we lied by saying "yes, we switched"). > We now say "sorry, we are already in zarch mode, sigp ignored" which also works > perfectly fine for Linux. And IMHO it is even the better choice even without > STFLE.138 being set as it matches what an old hardware would do when in zarch mode. Ok, if it worked for relevant Linux versions, than it should indeed be fine. Thanks! >> [...] >> >>> } >>> - return rc; >>> + >>> + *status_reg &= 0xffffffff00000000ULL; >> >> s/ULL/UL/ > > > Right, will fix. > -- Thanks, David -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
