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Date:   Thu, 10 Nov 2022 09:56:01 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Xin Li <xin3.li@intel.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, seanjc@google.com,
        pbonzini@redhat.com, kevin.tian@intel.com
Subject: Re: [RESEND PATCH 2/6] x86/traps: add a system interrupt table for
 system interrupt dispatch
Message-ID: <Y2y8obdYDXo9vlH/@hirez.programming.kicks-ass.net>
References: <20221110061545.1531-1-xin3.li@intel.com>
 <20221110061545.1531-3-xin3.li@intel.com>
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On Wed, Nov 09, 2022 at 10:15:41PM -0800, Xin Li wrote:
> Upon receiving an external interrupt, KVM VMX reinjects it through
> calling the interrupt handler in its IDT descriptor on the current
> kernel stack, which essentially uses the IDT as an interrupt dispatch
> table.
> 
> However the IDT is one of the lowest level critical data structures
> between a x86 CPU and the Linux kernel, we should avoid using it
> *directly* whenever possible, espeically in a software defined manner.
> 
> On x86, external interrupts are divided into the following groups
>   1) system interrupts
>   2) external device interrupts
> With the IDT, system interrupts are dispatched through the IDT
> directly, while external device interrupts are all routed to the
> external interrupt dispatch function common_interrupt(), which
> dispatches external device interrupts through a per-CPU external
> interrupt dispatch table vector_irq.
> 
> To eliminate dispatching external interrupts through the IDT, add
> a system interrupt handler table for dispatching a system interrupt
> to its corresponding handler directly. Thus a software based dispatch
> function will be:
> 
>   void external_interrupt(struct pt_regs *regs, u8 vector)
>   {
>     if (is_system_interrupt(vector))
>       system_interrupt_handler_table[vector_to_sysvec(vector)](regs);
>     else /* external device interrupt */
>       common_interrupt(regs, vector);
>   }
> 
> What's more, with the Intel FRED (Flexible Return and Event Delivery)
> architecture, IDT, the hardware based event dispatch table, is gone,
> and the Linux kernel needs to dispatch events to their handlers with
> vector to handler mappings, the dispatch function external_interrupt()
> is also needed.
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Signed-off-by: Xin Li <xin3.li@intel.com>

This is not a valid SOB, it would suggest hpa is the author, but he's
not in in From.

> ---
>  arch/x86/include/asm/traps.h |  8 ++++++
>  arch/x86/kernel/traps.c      | 55 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 63 insertions(+)
> 
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index 47ecfff2c83d..3dc63d753bda 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -47,4 +47,12 @@ void __noreturn handle_stack_overflow(struct pt_regs *regs,
>  				      struct stack_info *info);
>  #endif
>  
> +/*
> + * How system interrupt handlers are called.
> + */
> +#define DECLARE_SYSTEM_INTERRUPT_HANDLER(f)			\
> +	void f (struct pt_regs *regs __maybe_unused,		\
> +		unsigned long vector __maybe_unused)
> +typedef DECLARE_SYSTEM_INTERRUPT_HANDLER((*system_interrupt_handler));
> +
>  #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 178015a820f0..95dd917ef9ad 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -1444,6 +1444,61 @@ DEFINE_IDTENTRY_SW(iret_error)
>  }
>  #endif
>  
> +#define SYSV(x,y) [(x) - FIRST_SYSTEM_VECTOR] = (system_interrupt_handler)y
> +
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wcast-function-type"

How does this not break CFI ?