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

From: Fabiano Rosas
Subject: Re: [RFC PATCH 1/2] spapr: Report correct GTSE support via ov5
Date: Fri, 01 Apr 2022 12:50:49 -0300

"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:

> 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?

The spapr capability would mean "disable GTSE if KVM allows
it". Although I'd prefer using cap-gtse=<on/off> because it gives us
more flexibility if we ever want to change the default value.

On the KVM side I am testing a KVM_CAP_PPC_GTSE_DISABLE with the
semantics of "whether QEMU is allowed to disable GTSE". It reports the
inverse of MMU_FTR_GTSE. So if L1 runs with GTSE=0, then the capability
returns 1 and therefore QEMU can disable GTSE. If the capability is not
present, then QEMU is not allowed to disable GTSE.

With David's idea of disallowing cap-rpt-invalidate=off,cap-gtse=off we
can simply deny the nested guest command line if it doesn't include
cap-rpt-invalidate=on when KVM L1 reports KVM_CAP_PPC_GTSE_DISABLE. That
way cap-gtse can default to ON to keep TCG working.

On a first look, I think the above works. I'm still running some tests
with different QEMU/kernel versions.

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