Re: [PATCH 25/26] KVM: PPC: Book3S PR: Support TAR handling for PR KVM HTM.

On Thu, Jan 11, 2018 at 06:11:38PM +0800, wei.guo.simon@gmail.com wrote:

From: Simon Guo <redacted>

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.