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    Subject[RFC PATCH v3 27/32] x86/fred: allow single-step trap and NMI when starting a new thread
    Date
    From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

    Allow single-step trap and NMI when starting a new thread, thus once
    the new thread returns to ring3, single-step trap and NMI are both
    enabled immediately.

    High-order 48 bits above the lowest 16 bit CS are discarded by the
    legacy IRET instruction, thus can be set unconditionally, even when
    FRED is not enabled.

    Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
    Signed-off-by: Xin Li <xin3.li@intel.com>
    ---
    arch/x86/include/asm/fred.h | 11 +++++++++++
    arch/x86/kernel/process_64.c | 13 +++++++------
    2 files changed, 18 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
    index 6a417fc1e7ee..958751f16c94 100644
    --- a/arch/x86/include/asm/fred.h
    +++ b/arch/x86/include/asm/fred.h
    @@ -52,6 +52,14 @@
    #define FRED_CSL_ALLOW_SINGLE_STEP _BITUL(25)
    #define FRED_CSL_INTERRUPT_SHADOW _BITUL(24)

    +/*
    + * High-order 48 bits above the lowest 16 bit CS are discarded by the
    + * legacy IRET instruction, thus can be set unconditionally, even when
    + * FRED is not enabled.
    + */
    +#define CSL_PROCESS_START \
    + (FRED_CSL_ENABLE_NMI | FRED_CSL_ALLOW_SINGLE_STEP)
    +
    #ifndef __ASSEMBLY__

    #include <linux/kernel.h>
    @@ -115,6 +123,9 @@ void fred_setup_apic(void);
    #else
    #define cpu_init_fred_exceptions() BUG()
    #define fred_setup_apic() BUG()
    +
    +#define CSL_PROCESS_START 0
    +
    #endif /* CONFIG_X86_FRED */

    #endif /* ASM_X86_FRED_H */
    diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
    index ff6594dbea4a..ac3895f4a360 100644
    --- a/arch/x86/kernel/process_64.c
    +++ b/arch/x86/kernel/process_64.c
    @@ -55,6 +55,7 @@
    #include <asm/resctrl.h>
    #include <asm/unistd.h>
    #include <asm/fsgsbase.h>
    +#include <asm/fred.h>
    #ifdef CONFIG_IA32_EMULATION
    /* Not included via unistd.h */
    #include <asm/unistd_32_ia32.h>
    @@ -506,7 +507,7 @@ void x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)
    static void
    start_thread_common(struct pt_regs *regs, unsigned long new_ip,
    unsigned long new_sp,
    - unsigned int _cs, unsigned int _ss, unsigned int _ds)
    + u16 _cs, u16 _ss, u16 _ds)
    {
    WARN_ON_ONCE(regs != current_pt_regs());

    @@ -521,11 +522,11 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
    loadsegment(ds, _ds);
    load_gs_index(0);

    - regs->ip = new_ip;
    - regs->sp = new_sp;
    - regs->cs = _cs;
    - regs->ss = _ss;
    - regs->flags = X86_EFLAGS_IF;
    + regs->ip = new_ip;
    + regs->sp = new_sp;
    + regs->csl = _cs | CSL_PROCESS_START;
    + regs->ssl = _ss;
    + regs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
    }

    void
    --
    2.34.1
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