Re: [RFC PATCH 1/2] spapr: Report correct GTSE support via ov5

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

David Gibson <david@gibson.dropbear.id.au> writes:

> On Mon, Mar 14, 2022 at 07:10:10PM -0300, Fabiano Rosas wrote:
>> David Gibson <david@gibson.dropbear.id.au> writes:
>> 
>> > On Tue, Mar 08, 2022 at 10:23:59PM -0300, Fabiano Rosas wrote:
>>

...

>> To satisfy TCG we could keep a spapr capability as ON and usually the
>> guest would pass cap-gtse=off when running with KVM. However this
>> doesn't work because this crash happens precisely because the nested
>> guest doesn't know that it needs to use cap-rpt-invalidate=on. Another
>> cap wouldn't help.
>> 
>> So I think the only way to have a spapr capability for this is if TCG
>> always defaults to ON and KVM always defaults to OFF. But then we would
>> be changing guest visible behaviour depending on host properties.
>
> Ok, I'd forgotten we already have cap-rpt-invalidate.  It still
> defaults to OFF for now, which might help us.
>
> What's clear is that we should never disable GTSE if
> cap-rpt-invalidate is off - qemu should enforce that before even
> starting the guest if at all possible.
>
> What's less clear to me is if we want to enable GTSE by default or
> not, in the cases where we're able to choose.  Would always disabling
> GTSE when cap-rpt-invalidate=on be ok?  Or do we want to be able to
> control GTSE separately.  In that case we might need a second cap, but
> it would need inverted sense, so e.g. cap-disable-gtse.


GTSE and cap-rpt-invalidate can be looked at as independent such that we
can do GTSE=1 or GTSE=0 with cap-rpt-invalidate=on. But GTSE=0 with
cap-rpt-invalidate=off is not allowed/possible. GTSE value is what is
negotiated via CAS so we should let the hypervisor inform the guest whether it
can do GTSE 0 or 1. The challenge IIUC is Qemu always assumed GTSE=1
which is not true in the case of nested virt where L1 guest that is booted
with GTSE=0.

with cap-disable-gtse how would one interpret that? Whether hypervisor
have the capability to disable gtse? 

>
> I believe a guest that is expecting GTSE==0 should work if
> LPCR[GTSE]==1, just not optimally (as long as H_RPT_INVALIDATE is
> still available, of course).  Is that right?

That is correct.

-aneesh