"Aneesh Kumar K.V" [off-list ref] writes:

"Aneesh Kumar K.V" [off-list ref] writes:

quoted

Michael Neuling [off-list ref] writes:

quoted

Aneesh Kumar K.V [off-list ref] wrote:

quoted

Runtime disable transactional memory feature looking at pa-features
device tree entry. This provides a mechanism to disable TM on P8
systems.

What are we actually achieving with this?

PAPR compliance  :) ? Also I wanted to disable guest kernel from doing
TM related save restore. Guest kernel already look at the cpu feature
before doing that. Hence needed a mechanism to disable the feature. 

Things like

static inline void __switch_to_tm(struct task_struct *prev)
{
	if (cpu_has_feature(CPU_FTR_TM)) {
		tm_enable();
		tm_reclaim_task(prev);
	}
}

quoted

quoted

Signed-off-by: Aneesh Kumar K.V <redacted>
---
 arch/powerpc/kernel/prom.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
index 668aa4791fd7..537bd7e7db0b 100644
--- a/arch/powerpc/kernel/prom.c
+++ b/arch/powerpc/kernel/prom.c

@@ -161,6 +161,11 @@ static struct ibm_pa_feature {
 	{CPU_FTR_NODSISRALIGN, 0, 0,	1, 1, 1},
 	{0, MMU_FTR_CI_LARGE_PAGE, 0,	1, 2, 0},
 	{CPU_FTR_REAL_LE, PPC_FEATURE_TRUE_LE, 5, 0, 0},
+	/*
+	 * We should use CPU_FTR_TM_COMP so that if we disable TM, it won't get
+	 * enabled via device tree
+	 */
+	{CPU_FTR_TM_COMP, 0, 0,		22, 0, 0},

What does this do to guests?  Will it turn TM unavailable into an
illegal instruction?

Good suggestion. I guess it should be facility unavailable interrupt ?
I should also make the sure __init_HFSCR only set HFSCR_TM only if the
cpu feature is enabled ?

I looked at this and I guess we don't need to update HFSCR considering
that the guest kernel (privileged) access to TM always happen within
if (cpu_has_feature(CPU_FTR_TM)) conditional block. Also we want to
disable this per guest and there is no easy way to suggest hypervisor
that disable TM in HFSCR.

BTW we already do this for guest problme state. We do in guest kernel

	if (cpu_has_feature(CPU_FTR_TM))
		regs->msr |= MSR_TM;

IIUC that should result in facility unavailable interrupt when problem
state try to access TM ?

I will try to run some test with the patch and update here.

This will actually result in illegal instruction.

-aneesh