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

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Date:   Thu, 21 Sep 2023 12:48:17 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Xin Li <xin3.li@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-hyperv@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, jgross@...e.com, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, andrew.cooper3@...rix.com,
        jiangshanlai@...il.com
Subject: Re: [PATCH v10 28/38] x86/fred: FRED entry/exit and dispatch code



On 14.09.23 г. 7:47 ч., Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.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@...el.com>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
> Co-developed-by: Xin Li <xin3.li@...el.com>
> Tested-by: Shan Kang <shan.kang@...el.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Xin Li <xin3.li@...el.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>