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Re: [PATCH] ppc/xive: Save/restore state of the External interrupt

From: Cédric Le Goater
Subject: Re: [PATCH] ppc/xive: Save/restore state of the External interrupt
Date: Tue, 26 Apr 2022 14:35:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0

Hello Frederic,

On 4/26/22 12:11, Frederic Barrat wrote:
When pulling an OS context from a vcpu, we should lower the External
interrupt if it is pending. Otherwise, it may be delivered in the
hypervisor context, which is unexpected. It can lead to an infinite
loop where the hypervisor catches the External exception, looks for an
interrupt, doesn't find any, exits the exception handler, repeat...

Nice ! I have been chasing this one for a while and couldn't corner it.

Setting an environment for this scenario is a bit painful. I had a
pseries disk image including a nested guest for it. Depending on the
need, I would run the image under KVM (for updates) or under QEMU
PowerNV for dev/tests.

I thne switched to a simpler buildroot env.

It also means that when pushing a new OS context on a vcpu, we need to
always check the restored Interrupt Pending Buffer (IPB), potentially
merge it with any escalation interrupt, and re-apply the External
interrupt signal if needed.

See below for a proposal to split this patch in 2 or 3 patches.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>

Cedric: I'm wondering the reason behind commit
8256870ada9379abfd1f5b2c209ad01092dd0904, which makes the PIPR field
of the OS context read-only.

The comments is related to direct load/store on the TCTX registers.
When using the magic offsets, the logic is different.

The comment says it is re-evaluated from
the IPB when pushing a context, but I don't think it's true on P9
if there's no escalation. It's not a problem on P10 because we always
re-evaluate the PIPR if CPPR>0.
In any case, it's no longer an issue with this patch, but I'm
curious as to why restoring the PIPR was a problem in the first place.

  hw/intc/xive.c        | 26 +++++++++++++++++++++++---
  hw/intc/xive2.c       | 14 ++++++++------
  include/hw/ppc/xive.h |  1 +
  3 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/hw/intc/xive.c b/hw/intc/xive.c
index b8e4c7294d..8430db687f 100644
--- a/hw/intc/xive.c
+++ b/hw/intc/xive.c
@@ -114,6 +114,21 @@ static void xive_tctx_notify(XiveTCTX *tctx, uint8_t ring)
+void xive_tctx_eval_ext_signal(XiveTCTX *tctx)

I would call it xive_tctx_reset_os_signal()

+    uint8_t *regs = &tctx->regs[TM_QW1_OS];
+    /*
+     * Used when pulling an OS context.
+     * Lower the External interrupt if it's pending. It is necessary
+     * to avoid catching it in the hypervisor context. It should be
+     * raised again when re-pushing the OS context.
+     */
+    if (regs[TM_PIPR] < regs[TM_CPPR]) {

This seems useless. Since we want the signal down always.

+        qemu_irq_lower(xive_tctx_output(tctx, TM_QW1_OS));
+    }
  static void xive_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
      uint8_t *regs = &tctx->regs[ring];
@@ -388,6 +403,8 @@ static uint64_t xive_tm_pull_os_ctx(XivePresenter *xptr, 
XiveTCTX *tctx,
      /* Invalidate CAM line */
      qw1w2_new = xive_set_field32(TM_QW1W2_VO, qw1w2, 0);
      xive_tctx_set_os_cam(tctx, qw1w2_new);
+    xive_tctx_eval_ext_signal(tctx);

These change forcing the signal down when the OS context is pulled
should be in its own patch.

      return qw1w2;
@@ -413,10 +430,13 @@ static void xive_tctx_need_resend(XiveRouter *xrtr, XiveTCTX *tctx,
          /* Reset the NVT value */
          nvt.w4 = xive_set_field32(NVT_W4_IPB, nvt.w4, 0);
          xive_router_write_nvt(xrtr, nvt_blk, nvt_idx, &nvt, 4);
-        /* Merge in current context */
-        xive_tctx_ipb_update(tctx, TM_QW1_OS, ipb);
+    /*
+     * Always merge in current context. Even if there's no escalation,
+     * it will check with the IPB value restored before pushing the OS
+     * context and set the External interrupt signal if needed.
+     */
+    xive_tctx_ipb_update(tctx, TM_QW1_OS, ipb);

That's another patch and I am not sure it is needed.

An IPB value is recorded in the NVT when an interrupt is delivered
while a vCPU is not dispatched. With this change, you would force
the update in any case when the context is pushed.

Is that to close a potential race window on an interrupt being
delivered to a vCPU just before it is dispatched by the HV ?

diff --git a/hw/intc/xive2.c b/hw/intc/xive2.c
index 3aff42a69e..b7f1917cd2 100644
--- a/hw/intc/xive2.c
+++ b/hw/intc/xive2.c
@@ -269,6 +269,7 @@ uint64_t xive2_tm_pull_os_ctx(XivePresenter *xptr, XiveTCTX 
          xive2_tctx_save_os_ctx(xrtr, tctx, nvp_blk, nvp_idx);
+ xive_tctx_eval_ext_signal(tctx);
      return qw1w2;
@@ -316,7 +317,6 @@ static void xive2_tctx_need_resend(Xive2Router *xrtr, XiveTCTX *tctx,
      Xive2Nvp nvp;
      uint8_t ipb;
-    uint8_t cppr = 0;
       * Grab the associated thread interrupt context registers in the
@@ -337,7 +337,7 @@ static void xive2_tctx_need_resend(Xive2Router *xrtr, 
XiveTCTX *tctx,
      /* Automatically restore thread context registers */
      if (xive2_router_get_config(xrtr) & XIVE2_VP_SAVE_RESTORE &&
          do_restore) {
-        cppr = xive2_tctx_restore_os_ctx(xrtr, tctx, nvp_blk, nvp_idx, &nvp);
+        xive2_tctx_restore_os_ctx(xrtr, tctx, nvp_blk, nvp_idx, &nvp);
ipb = xive_get_field32(NVP2_W2_IPB, nvp.w2);
@@ -346,10 +346,12 @@ static void xive2_tctx_need_resend(Xive2Router *xrtr, 
XiveTCTX *tctx,
          xive2_router_write_nvp(xrtr, nvp_blk, nvp_idx, &nvp, 2);
- /* An IPB or CPPR change can trigger a resend */
-    if (ipb || cppr) {
-        xive_tctx_ipb_update(tctx, TM_QW1_OS, ipb);
-    }
+    /*
+     * Always merge in current context. Even if there's no escalation,
+     * it will check with the IPB value restored and set the External
+     * interrupt signal if needed.
+     */
+    xive_tctx_ipb_update(tctx, TM_QW1_OS, ipb);

I guess this fine. The test on the cppr is just an optimisation, if I am
correct. But it needs to be in another patch.

Same previous comment for ibp.



diff --git a/include/hw/ppc/xive.h b/include/hw/ppc/xive.h
index 126e4e2c3a..91512ed5e6 100644
--- a/include/hw/ppc/xive.h
+++ b/include/hw/ppc/xive.h
@@ -527,6 +527,7 @@ Object *xive_tctx_create(Object *cpu, XivePresenter *xptr, 
Error **errp);
  void xive_tctx_reset(XiveTCTX *tctx);
  void xive_tctx_destroy(XiveTCTX *tctx);
  void xive_tctx_ipb_update(XiveTCTX *tctx, uint8_t ring, uint8_t ipb);
+void xive_tctx_eval_ext_signal(XiveTCTX *tctx);
   * KVM XIVE device helpers

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