Re: [Qemu-devel] [PATCH v1 2/2] s390x/cpumodel: improve defintion search without an IBC

[David Hildenbrand]

On 06.06.2017 10:39, Christian Borntraeger wrote:
> On 06/02/2017 07:28 PM, Halil Pasic wrote:
> [...]
>> Maybe integrating some of the discussion from above into the commit
>> message would be helpful.
> 
> applied with the following patch description

Just wanted to ask you to do that :) Thanks!

Looks good to me!

> 
> 
> Currently, under z/VM on a 0x2827, QEMU will detect a 0x2828 if no
> IBC value is provided. QEMU will simply take the last model of that HW
> generation, which happens to be the BC version.
> 
> Let's improve our search for that case by selecting the latest CPU
> definition that matches the CPU type. This for example will avoid
> detecting an z13 as a z13s.
> 
> We might still detect a GA2 version on a GA1 system, but as we don't
> have further information at hand, there isn't too much we can do about
> it. The alternative of always presenting the oldest GA is not backward
> compatible, e.g:
> You're running on 0x2827 GA2.
> Old QEMU version indicated "0x2828 GA1 == 0x2827 GA2". After you updated
> QEMU, you suddenly detect "0x2827 GA1". You're previous libvirt guest
> might suddenly refuse to run.
> 
> In the end presenting a newer GA level does not matter because:
> 
> 1: All GAX models share the same base feature set. A GAX++ might
> support "more features".
> 2: Without an IBC, the guest can't detect the GA version.
> 
> If we have no IBC (esp. unblocked_ibc == 0), the IBC we will present
> to the guest in read_SCP_info() will be 0. The guest will not know
> which GA version it has. The problem of missing IBC propagates.
> 
> If we don't have a feature of the GA++ version, also our guest won't
> have it. So in summary, the guest also has no idea of its GA version.
> 
> Signed-off-by: David Hildenbrand <address@hidden>
> Message-Id: <address@hidden>
> Acked-by: Jason J. Herne <address@hidden>
> Reviewed-by: Halil Pasic <address@hidden>
> Signed-off-by: Christian Borntraeger <address@hidden>
> [improve patch description by reusing mailing list discussion]
> 
> 



-- 

Thanks,

David