linux-kernel - Re: [PATCH v3] xen: get rid of paravirt op adjust_exception

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Date:   Mon, 7 Aug 2017 16:56:52 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org
Cc:     hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
        luto@...capital.net
Subject: Re: [PATCH v3] xen: get rid of paravirt op adjust_exception_frame


> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 811e4ddb3f37..a3dcd83187ce 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -579,6 +579,71 @@ static void xen_write_ldt_entry(struct desc_struct *dt, int entrynum,
>  	preempt_enable();
>  }
>  
> +#ifdef CONFIG_X86_64
> +static struct {
> +	void (*orig)(void);
> +	void (*xen)(void);
> +	bool ist_okay;
> +	bool handle;
> +} trap_array[] = {
> +	{ debug, xen_xendebug, true, true },
> +	{ int3, xen_xenint3, true, true },
> +	{ double_fault, xen_double_fault, true, false },

Is it really worth adding 'handle' member to the structure because of a
single special case? We don't expect to ever have another such vector.

(TBH, I think current implementation of cvt_gate_to_trap() is clearer,
even if it is not as general as what is in this patch. I know that Andy
disagrees).

-boris

> +#ifdef CONFIG_X86_MCE
> +	{ machine_check, xen_machine_check, true, true },
> +#endif
> +	{ nmi, xen_nmi, true, true },
> +	{ overflow, xen_overflow, false, true },
> +#ifdef CONFIG_IA32_EMULATION
> +	{ entry_INT80_compat, xen_entry_INT80_compat, false, true },
> +#endif
> +	{ page_fault, xen_page_fault, false, true },
> +	{ divide_error, xen_divide_error, false, true },
> +	{ bounds, xen_bounds, false, true },
> +	{ invalid_op, xen_invalid_op, false, true },
> +	{ device_not_available, xen_device_not_available, false, true },
> +	{ coprocessor_segment_overrun, xen_coprocessor_segment_overrun,
> +	  false, true },
> +	{ invalid_TSS, xen_invalid_TSS, false, true },
> +	{ segment_not_present, xen_segment_not_present, false, true },
> +	{ stack_segment, xen_stack_segment, false, true },
> +	{ general_protection, xen_general_protection, false, true },
> +	{ spurious_interrupt_bug, xen_spurious_interrupt_bug, false, true },
> +	{ coprocessor_error, xen_coprocessor_error, false, true },
> +	{ alignment_check, xen_alignment_check, false, true },
> +	{ simd_coprocessor_error, xen_simd_coprocessor_error, false, true },
> +#ifdef CONFIG_TRACING
> +	{ trace_page_fault, xen_trace_page_fault, false, true },
> +#endif
> +};
> +
> +static bool get_trap_addr(unsigned long *addr, unsigned int ist)
> +{
> +	unsigned int nr;
> +	bool handle = true, ist_okay = false;
> +
> +	/*
> +	 * Replace trap handler addresses by Xen specific ones.
> +	 * Check for known traps using IST and whitelist them.
> +	 * The debugger ones are the only ones we care about.
> +	 * Xen will handle faults like double_fault, * so we should never see
> +	 * them.  Warn if there's an unexpected IST-using fault handler.
> +	 */
> +	for (nr = 0; nr < ARRAY_SIZE(trap_array); nr++)
> +		if (*addr == (unsigned long)trap_array[nr].orig) {
> +			*addr = (unsigned long)trap_array[nr].xen;
> +			ist_okay = trap_array[nr].ist_okay;
> +			handle = trap_array[nr].handle;
> +			break;
> +		}
> +
> +	if (WARN_ON(ist != 0 && !ist_okay))
> +		handle = false;
> +
> +	return handle;
> +}
> +#endif
> +
>  static int cvt_gate_to_trap(int vector, const gate_desc *val,
>  			    struct trap_info *info)
>  {
> @@ -591,40 +656,8 @@ static int cvt_gate_to_trap(int vector, const gate_desc *val,
>  
>  	addr = gate_offset(*val);
>  #ifdef CONFIG_X86_64
> -	/*
> -	 * Look for known traps using IST, and substitute them
> -	 * appropriately.  The debugger ones are the only ones we care
> -	 * about.  Xen will handle faults like double_fault,
> -	 * so we should never see them.  Warn if
> -	 * there's an unexpected IST-using fault handler.
> -	 */
> -	if (addr == (unsigned long)debug)
> -		addr = (unsigned long)xen_debug;
> -	else if (addr == (unsigned long)int3)
> -		addr = (unsigned long)xen_int3;
> -	else if (addr == (unsigned long)stack_segment)
> -		addr = (unsigned long)xen_stack_segment;
> -	else if (addr == (unsigned long)double_fault) {
> -		/* Don't need to handle these */
> +	if (!get_trap_addr(&addr, val->ist))
>  		return 0;
> -#ifdef CONFIG_X86_MCE
> -	} else if (addr == (unsigned long)machine_check) {
> -		/*
> -		 * when xen hypervisor inject vMCE to guest,
> -		 * use native mce handler to handle it
> -		 */
> -		;
> -#endif
> -	} else if (addr == (unsigned long)nmi)
> -		/*
> -		 * Use the native version as well.
> -		 */
> -		;
> -	else {
> -		/* Some other trap using IST? */
> -		if (WARN_ON(val->ist != 0))
> -			return 0;
> -	}
>  #endif	/* CONFIG_X86_64 */
>  	info->address = addr;
>