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ab31640 |
From: Andrew Cooper <andrew.cooper3@citrix.com>
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
Subject: x86/spec-ctrl: Rework SPEC_CTRL_ENTRY_FROM_INTR_IST
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ab31640 |
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ab31640 |
We are shortly going to add a conditional IBPB in this path.
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ab31640 |
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ab31640 |
Therefore, we cannot hold spec_ctrl_flags in %eax, and rely on only clobbering
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
it after we're done with its contents. %rbx is available for use, and the
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ab31640 |
more normal register to hold preserved information in.
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
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ab31640 |
With %rax freed up, use it instead of %rdx for the RSB tmp register, and for
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ab31640 |
the adjustment to spec_ctrl_flags.
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ab31640 |
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ab31640 |
This leaves no use of %rdx, except as 0 for the upper half of WRMSR. In
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ab31640 |
practice, %rdx is 0 from SAVE_ALL on all paths and isn't likely to change in
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ab31640 |
the foreseeable future, so update the macro entry requirements to state this
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ab31640 |
dependency. This marginal optimisation can be revisited if circumstances
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ab31640 |
change.
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ab31640 |
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ab31640 |
No practical change.
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ab31640 |
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ab31640 |
This is part of XSA-407.
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ab31640 |
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ab31640 |
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
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ab31640 |
Reviewed-by: Jan Beulich <jbeulich@suse.com>
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ab31640 |
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ab31640 |
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
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ab31640 |
index 2a86938f1f32..a1810bf4d311 100644
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ab31640 |
--- a/xen/arch/x86/x86_64/entry.S
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ab31640 |
+++ b/xen/arch/x86/x86_64/entry.S
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ab31640 |
@@ -932,7 +932,7 @@ ENTRY(double_fault)
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ab31640 |
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ab31640 |
GET_STACK_END(14)
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ab31640 |
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ab31640 |
- SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
+ SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, Clob: abcd */
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ab31640 |
/* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
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ab31640 |
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ab31640 |
mov STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rbx
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ab31640 |
@@ -968,7 +968,7 @@ handle_ist_exception:
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ab31640 |
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ab31640 |
GET_STACK_END(14)
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
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ab31640 |
- SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, Clob: acd */
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
+ SPEC_CTRL_ENTRY_FROM_INTR_IST /* Req: %rsp=regs, %r14=end, %rdx=0, Clob: abcd */
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ab31640 |
/* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
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ab31640 |
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ab31640 |
mov STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx
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ab31640 |
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h b/xen/include/asm-x86/spec_ctrl_asm.h
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ab31640 |
index 0ff1b118f882..15e24cde00d1 100644
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ab31640 |
--- a/xen/include/asm-x86/spec_ctrl_asm.h
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ab31640 |
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
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ab31640 |
@@ -251,34 +251,33 @@
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ab31640 |
*/
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ab31640 |
.macro SPEC_CTRL_ENTRY_FROM_INTR_IST
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
/*
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ab31640 |
- * Requires %rsp=regs, %r14=stack_end
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
- * Clobbers %rax, %rcx, %rdx
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ab31640 |
+ * Requires %rsp=regs, %r14=stack_end, %rdx=0
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ab31640 |
+ * Clobbers %rax, %rbx, %rcx, %rdx
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ab31640 |
*
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ab31640 |
* This is logical merge of DO_OVERWRITE_RSB and DO_SPEC_CTRL_ENTRY
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ab31640 |
* maybexen=1, but with conditionals rather than alternatives.
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ab31640 |
*/
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ab31640 |
- movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %eax
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
+ movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %ebx
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
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ab31640 |
- test $SCF_ist_rsb, %al
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ab31640 |
+ test $SCF_ist_rsb, %bl
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ab31640 |
jz .L\@_skip_rsb
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ab31640 |
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ab31640 |
- DO_OVERWRITE_RSB tmp=rdx /* Clobbers %rcx/%rdx */
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
+ DO_OVERWRITE_RSB /* Clobbers %rax/%rcx */
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ab31640 |
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ab31640 |
.L\@_skip_rsb:
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ab31640 |
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ab31640 |
- test $SCF_ist_sc_msr, %al
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ab31640 |
+ test $SCF_ist_sc_msr, %bl
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ab31640 |
jz .L\@_skip_msr_spec_ctrl
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ab31640 |
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ab31640 |
- xor %edx, %edx
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ab31640 |
+ xor %eax, %eax
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ab31640 |
testb $3, UREGS_cs(%rsp)
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ab31640 |
- setnz %dl
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ab31640 |
- not %edx
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ab31640 |
- and %dl, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
+ setnz %al
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ab31640 |
+ not %eax
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ab31640 |
+ and %al, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14)
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
/* Load Xen's intended value. */
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
mov $MSR_SPEC_CTRL, %ecx
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
movzbl STACK_CPUINFO_FIELD(xen_spec_ctrl)(%r14), %eax
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
- xor %edx, %edx
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
wrmsr
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
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![](https://seccdn.libravatar.org/avatar/f1a0ddfc363b14e167bf8548e95f340032d1964ae5b6ea4f235e351fd948008e?s=16&d=retro) |
ab31640 |
/* Opencoded UNLIKELY_START() with no condition. */
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