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Date: Wed, 24 Jan 2018 15:02:58 +1100
From: Paul Mackerras <paulus@ozlabs.org>
To: wei.guo.simon@gmail.com
Cc: linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org
Subject: Re: [PATCH 25/26] KVM: PPC: Book3S PR: Support TAR handling for PR
 KVM HTM.
Message-ID: <20180124040257.GA32139@fergus.ozlabs.ibm.com>
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On Thu, Jan 11, 2018 at 06:11:38PM +0800, wei.guo.simon@gmail.com wrote:
> From: Simon Guo <wei.guo.simon@gmail.com>
> 
> Currently guest kernel doesn't handle TAR fac unavailable and it always
> runs with TAR bit on. PR KVM will lazily enable TAR. TAR is not a
> frequent-use reg and it is not included in SVCPU struct.
> 
> To make it work for transaction memory at PR KVM:
> 1). Flush/giveup TAR at kvmppc_save_tm_pr().
> 2) If we are receiving a TAR fac unavail exception inside a transaction,
> the checkpointed TAR might be a TAR value from another process. So we need
> treclaim the transaction, then load the desired TAR value into reg, and
> perform trecheckpoint.
> 3) Load TAR facility at kvmppc_restore_tm_pr() when TM active.
> The reason we always loads TAR when restoring TM is that:
> If we don't do this way, when there is a TAR fac unavailable exception
> during TM active:
> case 1: it is the 1st TAR fac unavail exception after tbegin.
> vcpu->arch.tar should be reloaded as checkpoint tar val.
> case 2: it is the 2nd or later TAR fac unavail exception after tbegin.
> vcpu->arch.tar_tm should be reloaded as checkpoint tar val.
> There will be unnecessary difficulty to handle the above 2 cases.
> 
> at the end of emulating treclaim., the correct TAR val need to be loaded
> into reg if FSCR_TAR bit is on.
> at the beginning of emulating trechkpt., TAR needs to be flushed so that
> the right tar val can be copy into tar_tm.

Would it be simpler always to load up TAR when guest_MSR[TM] is 1?

Paul.