From: David Gibson <david@gibson.dropbear.id.au>
To: Paul Mackerras <paulus@ozlabs.org>
Cc: kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
"Cédric Le Goater" <clg@kaod.org>,
kvm@vger.kernel.org
Subject: Re: [PATCH] KVM: PPC: Book3S HV: Use feature flag CPU_FTR_P9_TIDR when accessing TIDR
Date: Tue, 21 Jul 2020 15:39:25 +1000 [thread overview]
Message-ID: <20200721053925.GG215446@umbus.fritz.box> (raw)
In-Reply-To: <20200721050445.GA3878639@thinks.paulus.ozlabs.org>
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On Tue, Jul 21, 2020 at 03:04:45PM +1000, Paul Mackerras wrote:
> On Tue, Jun 23, 2020 at 06:50:27PM +0200, Cédric Le Goater wrote:
> > The TIDR register is only available on POWER9 systems and code
> > accessing this register is not always protected by the CPU_FTR_P9_TIDR
> > flag. Fix that to make sure POWER10 systems won't use it as TIDR has
> > been removed.
>
> I'm concerned about what this patch would do if we are trying to
> migrate from a P9 guest to a guest on P10 in P9-compat mode, in that
> the destination QEMU would get an error on doing the SET_ONE_REG for
> the TIDR. I don't think the lack of TIDR is worth failing the
> migration for given that TIDR only actually does anything if you are
> using an accelerator, and KVM has never supported use of accelerators
> in guests. I'm cc'ing David Gibson for his comments on the
> compatibility and migration issues.
Having thought about this a bit more, I don't think it matters. We're
going to have to update qemu to handle POWER10 anyway. If this causes
a problem, this would just add one small thing to whatever we need to
fix there.
> In any case, given that both move to and move from TIDR will be no-ops
> on P10 (for privileged code), I don't think there is a great urgency
> for this patch.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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prev parent reply other threads:[~2020-07-21 5:41 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-23 16:50 [PATCH] KVM: PPC: Book3S HV: Use feature flag CPU_FTR_P9_TIDR when accessing TIDR Cédric Le Goater
2020-07-21 5:04 ` Paul Mackerras
2020-07-21 5:39 ` David Gibson [this message]
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