From: Paul Mackerras <paulus@ozlabs.org>
To: "Cédric Le Goater" <clg@kaod.org>
Cc: linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org,
kvm@vger.kernel.org, David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH] KVM: PPC: Book3S HV: Use feature flag CPU_FTR_P9_TIDR when accessing TIDR
Date: Tue, 21 Jul 2020 15:04:45 +1000 [thread overview]
Message-ID: <20200721050445.GA3878639@thinks.paulus.ozlabs.org> (raw)
In-Reply-To: <20200623165027.271215-1-clg@kaod.org>
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.
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.
Paul.
next prev parent reply other threads:[~2020-07-21 5:06 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 [this message]
2020-07-21 5:39 ` David Gibson
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