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linux/arch/x86/kernel/fred.c
Xin Li (Intel) fe85ee3919 x86/entry: Set FRED RSP0 on return to userspace instead of context switch
The FRED RSP0 MSR points to the top of the kernel stack for user level
event delivery. As this is the task stack it needs to be updated when a
task is scheduled in.

The update is done at context switch. That means it's also done when
switching to kernel threads, which is pointless as those never go out to
user space. For KVM threads this means there are two writes to FRED_RSP0 as
KVM has to switch to the guest value before VMENTER.

Defer the update to the exit to user space path and cache the per CPU
FRED_RSP0 value, so redundant writes can be avoided.

Provide fred_sync_rsp0() for KVM to keep the cache in sync with the actual
MSR value after returning from guest to host mode.

[ tglx: Massage change log ]

Suggested-by: Sean Christopherson <seanjc@google.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240822073906.2176342-4-xin@zytor.com
2024-08-25 19:23:00 +02:00

87 lines
3.0 KiB
C

/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/kernel.h>
#include <asm/desc.h>
#include <asm/fred.h>
#include <asm/tlbflush.h>
#include <asm/traps.h>
/* #DB in the kernel would imply the use of a kernel debugger. */
#define FRED_DB_STACK_LEVEL 1UL
#define FRED_NMI_STACK_LEVEL 2UL
#define FRED_MC_STACK_LEVEL 2UL
/*
* #DF is the highest level because a #DF means "something went wrong
* *while delivering an exception*." The number of cases for which that
* can happen with FRED is drastically reduced and basically amounts to
* "the stack you pointed me to is broken." Thus, always change stacks
* on #DF, which means it should be at the highest level.
*/
#define FRED_DF_STACK_LEVEL 3UL
#define FRED_STKLVL(vector, lvl) ((lvl) << (2 * (vector)))
DEFINE_PER_CPU(unsigned long, fred_rsp0);
EXPORT_PER_CPU_SYMBOL(fred_rsp0);
void cpu_init_fred_exceptions(void)
{
/* When FRED is enabled by default, remove this log message */
pr_info("Initialize FRED on CPU%d\n", smp_processor_id());
/*
* If a kernel event is delivered before a CPU goes to user level for
* the first time, its SS is NULL thus NULL is pushed into the SS field
* of the FRED stack frame. But before ERETS is executed, the CPU may
* context switch to another task and go to user level. Then when the
* CPU comes back to kernel mode, SS is changed to __KERNEL_DS. Later
* when ERETS is executed to return from the kernel event handler, a #GP
* fault is generated because SS doesn't match the SS saved in the FRED
* stack frame.
*
* Initialize SS to __KERNEL_DS when enabling FRED to avoid such #GPs.
*/
loadsegment(ss, __KERNEL_DS);
wrmsrl(MSR_IA32_FRED_CONFIG,
/* Reserve for CALL emulation */
FRED_CONFIG_REDZONE |
FRED_CONFIG_INT_STKLVL(0) |
FRED_CONFIG_ENTRYPOINT(asm_fred_entrypoint_user));
wrmsrl(MSR_IA32_FRED_STKLVLS, 0);
wrmsrl(MSR_IA32_FRED_RSP0, 0);
wrmsrl(MSR_IA32_FRED_RSP1, 0);
wrmsrl(MSR_IA32_FRED_RSP2, 0);
wrmsrl(MSR_IA32_FRED_RSP3, 0);
/* Enable FRED */
cr4_set_bits(X86_CR4_FRED);
/* Any further IDT use is a bug */
idt_invalidate();
/* Use int $0x80 for 32-bit system calls in FRED mode */
setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
}
/* Must be called after setup_cpu_entry_areas() */
void cpu_init_fred_rsps(void)
{
/*
* The purpose of separate stacks for NMI, #DB and #MC *in the kernel*
* (remember that user space faults are always taken on stack level 0)
* is to avoid overflowing the kernel stack.
*/
wrmsrl(MSR_IA32_FRED_STKLVLS,
FRED_STKLVL(X86_TRAP_DB, FRED_DB_STACK_LEVEL) |
FRED_STKLVL(X86_TRAP_NMI, FRED_NMI_STACK_LEVEL) |
FRED_STKLVL(X86_TRAP_MC, FRED_MC_STACK_LEVEL) |
FRED_STKLVL(X86_TRAP_DF, FRED_DF_STACK_LEVEL));
/* The FRED equivalents to IST stacks... */
wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB));
wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI));
wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF));
}