linux-kernel - [PATCH 5.15 04/89] x86/entry: Dont call error

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Date:   Fri, 22 Jul 2022 11:10:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Lai Jiangshan <jiangshan.ljs@...group.com>,
        Borislav Petkov <bp@...e.de>, Juergen Gross <jgross@...e.com>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Subject: [PATCH 5.15 04/89] x86/entry: Dont call error_entry() for XENPV

From: Lai Jiangshan <jiangshan.ljs@...group.com>

commit 64cbd0acb58203fb769ed2f4eab526d43e243847 upstream.

XENPV guests enter already on the task stack and they can't fault for
native_iret() nor native_load_gs_index() since they use their own pvop
for IRET and load_gs_index(). A CR3 switch is not needed either.

So there is no reason to call error_entry() in XENPV.

  [ bp: Massage commit message. ]

Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Juergen Gross <jgross@...e.com>
Link: https://lore.kernel.org/r/20220503032107.680190-6-jiangshanlai@gmail.com
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/entry/entry_64.S |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -333,8 +333,17 @@ SYM_CODE_END(push_and_clear_regs)
 	call push_and_clear_regs
 	UNWIND_HINT_REGS
 
-	call	error_entry
-	movq	%rax, %rsp			/* switch to the task stack if from userspace */
+	/*
+	 * Call error_entry() and switch to the task stack if from userspace.
+	 *
+	 * When in XENPV, it is already in the task stack, and it can't fault
+	 * for native_iret() nor native_load_gs_index() since XENPV uses its
+	 * own pvops for IRET and load_gs_index().  And it doesn't need to
+	 * switch the CR3.  So it can skip invoking error_entry().
+	 */
+	ALTERNATIVE "call error_entry; movq %rax, %rsp", \
+		"", X86_FEATURE_XENPV
+
 	ENCODE_FRAME_POINTER
 	UNWIND_HINT_REGS