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x86/process: Move the buffer clearing before MONITOR

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Move the VERW clearing before the MONITOR so that VERW doesn't disarm it
and the machine never enters C1.

Original idea by Kim Phillips <kim.phillips@amd.com>.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
This commit is contained in:
Borislav Petkov (AMD) 2025-04-14 15:33:19 +02:00
parent 2329f250e0
commit 8e786a85c0
2 changed files with 27 additions and 14 deletions
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25
arch/x86/include/asm/mwait.h
View File

@ -43,8 +43,6 @@ static __always_inline void __monitorx(const void *eax, u32 ecx, u32 edx)
static __always_inline void __mwait(u32 eax, u32 ecx)
{
x86_idle_clear_cpu_buffers();
/*
* Use the instruction mnemonic with implicit operands, as the LLVM
* assembler fails to assemble the mnemonic with explicit operands:
@ -98,7 +96,6 @@ static __always_inline void __mwaitx(u32 eax, u32 ebx, u32 ecx)
*/
static __always_inline void __sti_mwait(u32 eax, u32 ecx)
{
x86_idle_clear_cpu_buffers();
asm volatile("sti; mwait" :: "a" (eax), "c" (ecx));
}
@ -115,21 +112,29 @@ static __always_inline void __sti_mwait(u32 eax, u32 ecx)
*/
static __always_inline void mwait_idle_with_hints(u32 eax, u32 ecx)
{
if (need_resched())
return;
x86_idle_clear_cpu_buffers();
if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
const void *addr = &current_thread_info()->flags;
alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
__monitor(addr, 0, 0);
if (!need_resched()) {
if (ecx & 1) {
__mwait(eax, ecx);
} else {
__sti_mwait(eax, ecx);
raw_local_irq_disable();
}
if (need_resched())
goto out;
if (ecx & 1) {
__mwait(eax, ecx);
} else {
__sti_mwait(eax, ecx);
raw_local_irq_disable();
}
}
out:
current_clr_polling();
}

16
arch/x86/kernel/process.c
View File

@ -907,16 +907,24 @@ static __init bool prefer_mwait_c1_over_halt(void)
*/
static __cpuidle void mwait_idle(void)
{
if (need_resched())
return;
x86_idle_clear_cpu_buffers();
if (!current_set_polling_and_test()) {
const void *addr = &current_thread_info()->flags;
alternative_input("", "clflush (%[addr])", X86_BUG_CLFLUSH_MONITOR, [addr] "a" (addr));
__monitor(addr, 0, 0);
if (!need_resched()) {
__sti_mwait(0, 0);
raw_local_irq_disable();
}
if (need_resched())
goto out;
__sti_mwait(0, 0);
raw_local_irq_disable();
}
out:
__current_clr_polling();
}