Re: [PATCH v10 28/38] x86/fred: FRED entry/exit and dispatch code

Nikolay Borisov
2023-09-21T21:08:03Z

On 14.09.23 г. 7:47 ч., Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>
> The code to actually handle kernel and event entry/exit using
> FRED. It is split up into two files thus:
>
> - entry_64_fred.S contains the actual entrypoints and exit code, and
> saves and restores registers.
> - entry_fred.c contains the two-level event dispatch code for FRED.
> The first-level dispatch is on the event type, and the second-level
> is on the event vector.
>
> Originally-by: Megha Dey <megha.dey@intel.com>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Co-developed-by: Xin Li <xin3.li@intel.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---
>
> Changes since v9:
> * Don't use jump tables, indirect jumps are expensive (Thomas Gleixner).
> * Except NMI/#DB/#MCE, FRED really can share the exception handlers
> with IDT (Thomas Gleixner).
> * Avoid the sysvec_* idt_entry muck, do it at a central place, reuse code
> instead of blindly copying it, which breaks the performance optimized
> sysvec entries like reschedule_ipi (Thomas Gleixner).
> * Add asm_ prefix to FRED asm entry points (Thomas Gleixner).
>
> Changes since v8:
> * Don't do syscall early out in fred_entry_from_user() before there are
> proper performance numbers and justifications (Thomas Gleixner).
> * Add the control exception handler to the FRED exception handler table
> (Thomas Gleixner).
> * Add ENDBR to the FRED_ENTER asm macro.
> * Reflect the FRED spec 5.0 change that ERETS and ERETU add 8 to %rsp
> before popping the return context from the stack.
>
> Changes since v1:
> * Initialize a FRED exception handler to fred_bad_event() instead of NULL
> if no FRED handler defined for an exception vector (Peter Zijlstra).
> * Push calling irqentry_{enter,exit}() and instrumentation_{begin,end}()
> down into individual FRED exception handlers, instead of in the dispatch
> framework (Peter Zijlstra).
> ---
> arch/x86/entry/Makefile | 5 +-
> arch/x86/entry/entry_64_fred.S | 52 ++++++
> arch/x86/entry/entry_fred.c | 230 ++++++++++++++++++++++++++
> arch/x86/include/asm/asm-prototypes.h | 1 +
> arch/x86/include/asm/fred.h | 6 +
> 5 files changed, 293 insertions(+), 1 deletion(-)
> create mode 100644 arch/x86/entry/entry_64_fred.S
> create mode 100644 arch/x86/entry/entry_fred.c
>

<snip>

> +
> +static noinstr void fred_intx(struct pt_regs *regs)
> +{
> + switch (regs->fred_ss.vector) {
> + /* INT0 */
> + case X86_TRAP_OF:
> + exc_overflow(regs);
> + return;
> +
> + /* INT3 */
> + case X86_TRAP_BP:
> + exc_int3(regs);
> + return;
> +
> + /* INT80 */
> + case IA32_SYSCALL_VECTOR:
> + if (likely(IS_ENABLED(CONFIG_IA32_EMULATION))) {

Since future kernels will support boottime toggling of whether 32bit
syscall interface should be enabled or not as per:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/entry&id=1da5c9bc119d3a749b519596b93f9b2667e93c4a

It will make more sense to replace this with ia32_enabled() invocation.
I guess this could be done as a follow-up patch based on when this is
merged as the ia32_enbaled changes are going to be merged in 6.7.

> + /* Save the syscall number */
> + regs->orig_ax = regs->ax;
> + regs->ax = -ENOSYS;
> + do_int80_syscall_32(regs);
> + return;
> + }
> + fallthrough;
> +
> + default:
> + exc_general_protection(regs, 0);
> + return;
> + }
> +}
> +
> +static __always_inline void fred_other(struct pt_regs *regs)
> +{
> + /* The compiler can fold these conditions into a single test */
> + if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.lm)) {
> + regs->orig_ax = regs->ax;
> + regs->ax = -ENOSYS;
> + do_syscall_64(regs, regs->orig_ax);
> + return;
> + } else if (likely(IS_ENABLED(CONFIG_IA32_EMULATION) &&

Ditto

> + regs->fred_ss.vector == FRED_SYSENTER &&
> + !regs->fred_ss.lm)) {
> + regs->orig_ax = regs->ax;
> + regs->ax = -ENOSYS;
> + do_fast_syscall_32(regs);
> + return;
> + } else {
> + exc_invalid_op(regs);
> + return;
> + }
> +}
> +

<snip>