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x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

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Decouple the use of ALTERNATIVE from the encoding of VERW to clear CPU
buffers so that KVM can use ALTERNATIVE_2 to handle "always clear buffers"
and "clear if guest can access host MMIO" in a single statement.

No functional change intended.

Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://patch.msgid.link/20251113233746.1703361-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-11-13 15:37:40 -08:00
parent aba7de6088
commit afb99ffbd5
1 changed files with 16 additions and 11 deletions
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27
arch/x86/include/asm/nospec-branch.h
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@@ -308,24 +308,29 @@
* CFLAGS.ZF.
* Note: Only the memory operand variant of VERW clears the CPU buffers.
*/
.macro __CLEAR_CPU_BUFFERS feature
#ifdef CONFIG_X86_64
ALTERNATIVE "", "verw x86_verw_sel(%rip)", \feature
#define VERW verw x86_verw_sel(%rip)
#else
/*
* In 32bit mode, the memory operand must be a %cs reference. The data
* segments may not be usable (vm86 mode), and the stack segment may not
* be flat (ESPFIX32).
*/
ALTERNATIVE "", "verw %cs:x86_verw_sel", \feature
/*
* In 32bit mode, the memory operand must be a %cs reference. The data segments
* may not be usable (vm86 mode), and the stack segment may not be flat (ESPFIX32).
*/
#define VERW verw %cs:x86_verw_sel
#endif
.endm
/*
* Provide a stringified VERW macro for simple usage, and a non-stringified
* VERW macro for use in more elaborate sequences, e.g. to encode a conditional
* VERW within an ALTERNATIVE.
*/
#define __CLEAR_CPU_BUFFERS __stringify(VERW)
/* If necessary, emit VERW on exit-to-userspace to clear CPU buffers. */
#define CLEAR_CPU_BUFFERS \
__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
#define VM_CLEAR_CPU_BUFFERS \
__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF_VM
ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
#ifdef CONFIG_X86_64
.macro CLEAR_BRANCH_HISTORY