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From 2a9e690a0ad5d54dca4166e089089a07bbe7fc85 Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Fri, 1 Jul 2022 15:59:40 +0100
Subject: [PATCH 47/51] x86/spec-ctrl: Rework SPEC_CTRL_ENTRY_FROM_INTR_IST
We are shortly going to add a conditional IBPB in this path.
Therefore, we cannot hold spec_ctrl_flags in %eax, and rely on only clobbering
it after we're done with its contents. %rbx is available for use, and the
more normal register to hold preserved information in.
With %rax freed up, use it instead of %rdx for the RSB tmp register, and for
the adjustment to spec_ctrl_flags.
This leaves no use of %rdx, except as 0 for the upper half of WRMSR. In
practice, %rdx is 0 from SAVE_ALL on all paths and isn't likely to change in
the foreseeable future, so update the macro entry requirements to state this
dependency. This marginal optimisation can be revisited if circumstances
change.
No practical change.
This is part of XSA-407.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit e9b8d31981f184c6539f91ec54bd9cae29cdae36)
---
xen/arch/x86/x86_64/entry.S | 4 ++--
xen/include/asm-x86/spec_ctrl_asm.h | 21 ++++++++++-----------
2 files changed, 12 insertions(+), 13 deletions(-)
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 2a86938f1f32..a1810bf4d311 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -932,7 +932,7 @@ ENTRY(double_fault)
GET_STACK_END(14)
- SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
+ SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, Clob: abcd */
/* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
mov STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
@@ -968,7 +968,7 @@ handle_ist_exception:
GET_STACK_END(14)
- SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
+ SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, Clob: abcd */
/* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
mov STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h b/xen/include/asm-x86/spec_ctrl_asm.h
index 0ff1b118f882..15e24cde00d1 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -251,34 +251,33 @@
*/
.macro SPEC_CTRL_ENTRY_FROM_INTR_IST
/*
- * Requires %rsp=regs, %r14=stack_end
- * Clobbers %rax, %rcx, %rdx
+ * Requires %rsp=regs, %r14=stack_end, %rdx=0
+ * Clobbers %rax, %rbx, %rcx, %rdx
*
* This is logical merge of DO_OVERWRITE_RSB and DO_SPEC_CTRL_ENTRY
* maybexen=1, but with conditionals rather than alternatives.
*/
- movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %eax
+ movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %ebx
- test $SCF_ist_rsb, %al
+ test $SCF_ist_rsb, %bl
jz .L\@_skip_rsb
- DO_OVERWRITE_RSB tmp=rdx /* Clobbers %rcx/%rdx */
+ DO_OVERWRITE_RSB /* Clobbers %rax/%rcx */
.L\@_skip_rsb:
- test $SCF_ist_sc_msr, %al
+ test $SCF_ist_sc_msr, %bl
jz .L\@_skip_msr_spec_ctrl
- xor %edx, %edx
+ xor %eax, %eax
testb $3, UREGS_cs(%rsp)
- setnz %dl
- not %edx
- and %dl, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
+ setnz %al
+ not %eax
+ and %al, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
/* Load Xen's intended value. */
mov $MSR_SPEC_CTRL, %ecx
movzbl STACK_CPUINFO_FIELD(xen_spec_ctrl)(%r14), %eax
- xor %edx, %edx
wrmsr
/* Opencoded UNLIKELY_START() with no condition. */
--
2.35.1
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