linux-kernel - [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

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Message-Id: <20191018161033.261971-4-samitolvanen@google.com>
Date:   Fri, 18 Oct 2019 09:10:18 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 03/18] arm64: kvm: stop treating register x18 as caller save

From: Ard Biesheuvel <ard.biesheuvel@...aro.org>

In preparation of using x18 as a task struct pointer register when
running in the kernel, stop treating it as caller save in the KVM
guest entry/exit code. Currently, the code assumes there is no need
to preserve it for the host, given that it would have been assumed
clobbered anyway by the function call to __guest_enter(). Instead,
preserve its value and restore it upon return.

Link: https://patchwork.kernel.org/patch/9836891/
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 arch/arm64/kvm/hyp/entry.S | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index e5cc8d66bf53..20bd9a20ea27 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -23,6 +23,7 @@
 	.pushsection	.hyp.text, "ax"
 
 .macro save_callee_saved_regs ctxt
+	str	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 	stp	x19, x20, [\ctxt, #CPU_XREG_OFFSET(19)]
 	stp	x21, x22, [\ctxt, #CPU_XREG_OFFSET(21)]
 	stp	x23, x24, [\ctxt, #CPU_XREG_OFFSET(23)]
@@ -38,6 +39,7 @@
 	ldp	x25, x26, [\ctxt, #CPU_XREG_OFFSET(25)]
 	ldp	x27, x28, [\ctxt, #CPU_XREG_OFFSET(27)]
 	ldp	x29, lr,  [\ctxt, #CPU_XREG_OFFSET(29)]
+	ldr	x18,      [\ctxt, #CPU_XREG_OFFSET(18)]
 .endm
 
 /*
@@ -87,12 +89,9 @@ alternative_else_nop_endif
 	ldp	x14, x15, [x18, #CPU_XREG_OFFSET(14)]
 	ldp	x16, x17, [x18, #CPU_XREG_OFFSET(16)]
 
-	// Restore guest regs x19-x29, lr
+	// Restore guest regs x18-x29, lr
 	restore_callee_saved_regs x18
 
-	// Restore guest reg x18
-	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]
-
 	// Do not touch any register after this!
 	eret
 	sb
@@ -114,7 +113,7 @@ ENTRY(__guest_exit)
 	// Retrieve the guest regs x0-x1 from the stack
 	ldp	x2, x3, [sp], #16	// x0, x1
 
-	// Store the guest regs x0-x1 and x4-x18
+	// Store the guest regs x0-x1 and x4-x17
 	stp	x2, x3,   [x1, #CPU_XREG_OFFSET(0)]
 	stp	x4, x5,   [x1, #CPU_XREG_OFFSET(4)]
 	stp	x6, x7,   [x1, #CPU_XREG_OFFSET(6)]
@@ -123,9 +122,8 @@ ENTRY(__guest_exit)
 	stp	x12, x13, [x1, #CPU_XREG_OFFSET(12)]
 	stp	x14, x15, [x1, #CPU_XREG_OFFSET(14)]
 	stp	x16, x17, [x1, #CPU_XREG_OFFSET(16)]
-	str	x18,      [x1, #CPU_XREG_OFFSET(18)]
 
-	// Store the guest regs x19-x29, lr
+	// Store the guest regs x18-x29, lr
 	save_callee_saved_regs x1
 
 	get_host_ctxt	x2, x3
-- 
2.23.0.866.gb869b98d4c-goog