Re: [PATCH] x86/xen: support early interrupts in xen pv guests

From: Andy Lutomirski
Date: Fri Nov 24 2017 - 10:52:20 EST

On Fri, Nov 24, 2017 at 12:52 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
> 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.

Nice! It would be great to get this into 4.15.

>>
>> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>> 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:
>>
>