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Date:   Fri, 24 Nov 2017 09:52:47 +0100
From:   Juergen Gross <jgross@...e.com>
To:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org
Cc:     boris.ostrovsky@...cle.com, hpa@...or.com, tglx@...utronix.de,
        mingo@...hat.com, Andrew Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/xen: support early interrupts in xen pv guests

Sorry, Andy, forgot to Cc: you...

On 24/11/17 09:42, Juergen Gross wrote:
> Add early interrupt handlers activated by idt_setup_early_handler() to
> the handlers supported by Xen pv guests. This will allow for early
> WARN() calls not crashing the guest.
> 
> Suggested-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/include/asm/segment.h | 12 ++++++++++++
>  arch/x86/mm/extable.c          |  4 +++-
>  arch/x86/xen/enlighten_pv.c    | 37 ++++++++++++++++++++++++-------------
>  arch/x86/xen/xen-asm_64.S      | 14 ++++++++++++++
>  4 files changed, 53 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
> index b20f9d623f9c..8f09012b92e7 100644
> --- a/arch/x86/include/asm/segment.h
> +++ b/arch/x86/include/asm/segment.h
> @@ -236,11 +236,23 @@
>   */
>  #define EARLY_IDT_HANDLER_SIZE 9
>  
> +/*
> + * xen_early_idt_handler_array is for Xen pv guests: for each entry in
> + * early_idt_handler_array it contains a prequel in the form of
> + * pop %rcx; pop %r11; jmp early_idt_handler_array[i]; summing up to
> + * max 8 bytes.
> + */
> +#define XEN_EARLY_IDT_HANDLER_SIZE 8
> +
>  #ifndef __ASSEMBLY__
>  
>  extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_SIZE];
>  extern void early_ignore_irq(void);
>  
> +#if defined(CONFIG_X86_64) && defined(CONFIG_XEN_PV)
> +extern const char xen_early_idt_handler_array[NUM_EXCEPTION_VECTORS][XEN_EARLY_IDT_HANDLER_SIZE];
> +#endif
> +
>  /*
>   * Load a segment. Fall back on loading the zero segment if something goes
>   * wrong.  This variant assumes that loading zero fully clears the segment.
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 3321b446b66c..88754bfd425f 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -1,6 +1,7 @@
>  #include <linux/extable.h>
>  #include <linux/uaccess.h>
>  #include <linux/sched/debug.h>
> +#include <xen/xen.h>
>  
>  #include <asm/fpu/internal.h>
>  #include <asm/traps.h>
> @@ -212,8 +213,9 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
>  	 * Old CPUs leave the high bits of CS on the stack
>  	 * undefined.  I'm not sure which CPUs do this, but at least
>  	 * the 486 DX works this way.
> +	 * Xen pv domains are not using the default __KERNEL_CS.
>  	 */
> -	if (regs->cs != __KERNEL_CS)
> +	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
>  		goto fail;
>  
>  	/*
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 5b2b3f3f6531..f2414c6c5e7c 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -622,7 +622,7 @@ static struct trap_array_entry trap_array[] = {
>  	{ simd_coprocessor_error,      xen_simd_coprocessor_error,      false },
>  };
>  
> -static bool get_trap_addr(void **addr, unsigned int ist)
> +static bool __ref get_trap_addr(void **addr, unsigned int ist)
>  {
>  	unsigned int nr;
>  	bool ist_okay = false;
> @@ -644,6 +644,14 @@ static bool get_trap_addr(void **addr, unsigned int ist)
>  		}
>  	}
>  
> +	if (nr == ARRAY_SIZE(trap_array) &&
> +	    *addr >= (void *)early_idt_handler_array[0] &&
> +	    *addr < (void *)early_idt_handler_array[NUM_EXCEPTION_VECTORS]) {
> +		nr = (*addr - (void *)early_idt_handler_array[0]) /
> +		     EARLY_IDT_HANDLER_SIZE;
> +		*addr = (void *)xen_early_idt_handler_array[nr];
> +	}
> +
>  	if (WARN_ON(ist != 0 && !ist_okay))
>  		return false;
>  
> @@ -1262,6 +1270,21 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  	xen_setup_gdt(0);
>  
>  	xen_init_irq_ops();
> +
> +	/* Let's presume PV guests always boot on vCPU with id 0. */
> +	per_cpu(xen_vcpu_id, 0) = 0;
> +
> +	/*
> +	 * Setup xen_vcpu early because idt_setup_early_handler needs it for
> +	 * local_irq_disable(), irqs_disabled().
> +	 *
> +	 * Don't do the full vcpu_info placement stuff until we have
> +	 * the cpu_possible_mask and a non-dummy shared_info.
> +	 */
> +	xen_vcpu_info_reset(0);
> +
> +	idt_setup_early_handler();
> +
>  	xen_init_capabilities();
>  
>  #ifdef CONFIG_X86_LOCAL_APIC
> @@ -1295,18 +1318,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  	 */
>  	acpi_numa = -1;
>  #endif
> -	/* Let's presume PV guests always boot on vCPU with id 0. */
> -	per_cpu(xen_vcpu_id, 0) = 0;
> -
> -	/*
> -	 * Setup xen_vcpu early because start_kernel needs it for
> -	 * local_irq_disable(), irqs_disabled().
> -	 *
> -	 * Don't do the full vcpu_info placement stuff until we have
> -	 * the cpu_possible_mask and a non-dummy shared_info.
> -	 */
> -	xen_vcpu_info_reset(0);
> -
>  	WARN_ON(xen_cpuhp_setup(xen_cpu_up_prepare_pv, xen_cpu_dead_pv));
>  
>  	local_irq_disable();
> diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
> index 8a10c9a9e2b5..417b339e5c8e 100644
> --- a/arch/x86/xen/xen-asm_64.S
> +++ b/arch/x86/xen/xen-asm_64.S
> @@ -15,6 +15,7 @@
>  
>  #include <xen/interface/xen.h>
>  
> +#include <linux/init.h>
>  #include <linux/linkage.h>
>  
>  .macro xen_pv_trap name
> @@ -54,6 +55,19 @@ xen_pv_trap entry_INT80_compat
>  #endif
>  xen_pv_trap hypervisor_callback
>  
> +	__INIT
> +ENTRY(xen_early_idt_handler_array)
> +	i = 0
> +	.rept NUM_EXCEPTION_VECTORS
> +	pop %rcx
> +	pop %r11
> +	jmp early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE
> +	i = i + 1
> +	.fill xen_early_idt_handler_array + i*XEN_EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
> +	.endr
> +END(xen_early_idt_handler_array)
> +	__FINIT
> +
>  hypercall_iret = hypercall_page + __HYPERVISOR_iret * 32
>  /*
>   * Xen64 iret frame:
> 

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