67 lines
2.7 KiB
Diff
67 lines
2.7 KiB
Diff
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From 51846ff74e3352151f99cfcfbe091c09f3ec8097 Mon Sep 17 00:00:00 2001
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From: Michael Brown <mcb30@ipxe.org>
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Date: Tue, 9 May 2023 12:09:30 +0000
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Subject: [PATCH 16/18] OvmfPkg: Clarify invariants for NestedInterruptTplLib
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NestedInterruptTplLib relies on CPU interrupts being disabled to
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guarantee exclusive (and hence atomic) access to the shared state in
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IsrState. Nothing in the calling interrupt handler should have
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re-enabled interrupts before calling NestedInterruptRestoreTPL(), and
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the loop in NestedInterruptRestoreTPL() itself maintains the invariant
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that interrupts are disabled at the start of each iteration.
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Add assertions to clarify this invariant, and expand the comments
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around the calls to RestoreTPL() and DisableInterrupts() to clarify
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the expectations around enabling and disabling interrupts.
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Signed-off-by: Michael Brown <mcb30@ipxe.org>
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Acked-by: Laszlo Ersek <lersek@redhat.com>
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(cherry picked from commit ae0be176a83efebe9a8c13d2124151f7dd13443a)
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---
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OvmfPkg/Library/NestedInterruptTplLib/Tpl.c | 10 ++++++++--
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1 file changed, 8 insertions(+), 2 deletions(-)
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diff --git a/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c b/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
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index e19d98878eb7..e921a09c5599 100644
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--- a/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
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+++ b/OvmfPkg/Library/NestedInterruptTplLib/Tpl.c
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@@ -104,6 +104,7 @@ NestedInterruptRestoreTPL (
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// defer our call to RestoreTPL() to the in-progress outer instance
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// of the same interrupt handler.
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//
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+ ASSERT (GetInterruptState () == FALSE);
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if (InterruptedTPL == IsrState->InProgressRestoreTPL) {
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//
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// Trigger outer instance of this interrupt handler to perform the
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@@ -153,6 +154,7 @@ NestedInterruptRestoreTPL (
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//
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// Check shared state loop invariants.
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//
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+ ASSERT (GetInterruptState () == FALSE);
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ASSERT (IsrState->InProgressRestoreTPL < InterruptedTPL);
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ASSERT (IsrState->DeferredRestoreTPL == FALSE);
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@@ -167,13 +169,17 @@ NestedInterruptRestoreTPL (
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//
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// Call RestoreTPL() to allow event notifications to be
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- // dispatched. This will implicitly re-enable interrupts.
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+ // dispatched. This will implicitly re-enable interrupts (if
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+ // InterruptedTPL is below TPL_HIGH_LEVEL), even though we are
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+ // still inside the interrupt handler.
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//
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gBS->RestoreTPL (InterruptedTPL);
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//
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// Re-disable interrupts after the call to RestoreTPL() to ensure
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- // that we have exclusive access to the shared state.
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+ // that we have exclusive access to the shared state. Interrupts
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+ // will be re-enabled by the IRET or equivalent instruction when
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+ // we subsequently return from the interrupt handler.
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//
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DisableInterrupts ();
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--
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2.40.1
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