Re: [PATCH v2] xen: get rid of paravirt op adjust_exception_frame

From: Juergen Gross
Date: Wed Aug 02 2017 - 07:08:53 EST

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On 01/08/17 21:45, Andy Lutomirski wrote:
> On Tue, Aug 1, 2017 at 3:39 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
>> When running as Xen pv-guest the exception frame on the stack contains
>> %r11 and %rcx additional to the other data pushed by the processor.
>>
>> Instead of having a paravirt op being called for each exception type
>> prepend the Xen specific code to each exception entry. When running as
>> Xen pv-guest just use the exception entry with prepended instructions,
>> otherwise use the entry without the Xen specific code.
>>
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>> arch/x86/entry/entry_64.S | 22 ++------------
>> arch/x86/entry/entry_64_compat.S | 1 -
>> arch/x86/include/asm/desc.h | 16 ++++++++++
>> arch/x86/include/asm/paravirt.h | 5 ---
>> arch/x86/include/asm/paravirt_types.h | 4 ---
>> arch/x86/include/asm/proto.h | 3 ++
>> arch/x86/include/asm/traps.h | 51 +++++++++++++++++++++++++++++--
>> arch/x86/kernel/asm-offsets_64.c | 1 -
>> arch/x86/kernel/paravirt.c | 3 --
>> arch/x86/kernel/traps.c | 57 ++++++++++++++++++-----------------
>> arch/x86/xen/enlighten_pv.c | 16 +++++-----
>> arch/x86/xen/irq.c | 3 --
>> arch/x86/xen/xen-asm_64.S | 45 ++++++++++++++++++++++++---
>> arch/x86/xen/xen-ops.h | 1 -
>> 14 files changed, 147 insertions(+), 81 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index a9a8027a6c0e..602bcf68a32c 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -745,7 +745,6 @@ ENTRY(\sym)
>> .endif
>>
>> ASM_CLAC
>> - PARAVIRT_ADJUST_EXCEPTION_FRAME
>>
>> .ifeq \has_error_code
>> pushq $-1 /* ORIG_RAX: no syscall to restart */
>> @@ -901,7 +900,7 @@ ENTRY(do_softirq_own_stack)
>> END(do_softirq_own_stack)
>>
>> #ifdef CONFIG_XEN
>> -idtentry xen_hypervisor_callback xen_do_hypervisor_callback has_error_code=0
>> +idtentry hypervisor_callback xen_do_hypervisor_callback has_error_code=0
>>
>> /*
>> * A note on the "critical region" in our callback handler.
>> @@ -967,8 +966,6 @@ ENTRY(xen_failsafe_callback)
>> movq 8(%rsp), %r11
>> addq $0x30, %rsp
>> pushq $0 /* RIP */
>> - pushq %r11
>> - pushq %rcx
>> jmp general_protection
>> 1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
>> movq (%rsp), %rcx
>> @@ -997,9 +994,8 @@ idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
>> idtentry stack_segment do_stack_segment has_error_code=1
>>
>> #ifdef CONFIG_XEN
>> -idtentry xen_debug do_debug has_error_code=0
>> -idtentry xen_int3 do_int3 has_error_code=0
>> -idtentry xen_stack_segment do_stack_segment has_error_code=1
>> +idtentry xendebug do_debug has_error_code=0
>> +idtentry xenint3 do_int3 has_error_code=0
>> #endif
>>
>> idtentry general_protection do_general_protection has_error_code=1
>> @@ -1161,18 +1157,6 @@ END(error_exit)
>> /* Runs on exception stack */
>> ENTRY(nmi)
>> /*
>> - * Fix up the exception frame if we're on Xen.
>> - * PARAVIRT_ADJUST_EXCEPTION_FRAME is guaranteed to push at most
>> - * one value to the stack on native, so it may clobber the rdx
>> - * scratch slot, but it won't clobber any of the important
>> - * slots past it.
>> - *
>> - * Xen is a different story, because the Xen frame itself overlaps
>> - * the "NMI executing" variable.
>> - */
>
> Based on Andrew Cooper's email, it sounds like this function is just
> straight-up broken on Xen PV. Maybe change the comment to "XXX:
> broken on Xen PV" or similar.

Fine with me.

>
>> +#if defined(CONFIG_X86_64) && defined(CONFIG_XEN_PV)
>> +#define pv_trap_entry(name) (void *)(xen_pv_domain() ? xen_ ## name : name)
>> +#else
>> +#define pv_trap_entry(name) (void *)(name)
>> +#endif
>> +
>
> Seems reasonable to me.
>
>> #ifdef CONFIG_X86_64
>>
>> static inline void pack_gate(gate_desc *gate, unsigned type, unsigned long func,
>> @@ -482,6 +490,14 @@ static inline void _set_gate(int gate, unsigned type, void *addr,
>> 0, 0, __KERNEL_CS); \
>> } while (0)
>>
>> +#define set_intr_gate_pv(n, addr) \
>> + do { \
>> + set_intr_gate_notrace(n, pv_trap_entry(addr)); \
>> + _trace_set_gate(n, GATE_INTERRUPT, \
>> + pv_trap_entry(trace_##addr), \
>> + 0, 0, __KERNEL_CS); \
>> + } while (0)
>
> Any reason this can't be set_intr_gate((n), pv_trap_entry(addr))? Or
> does that get preprocessed wrong?

trace_##addr won't look like anything the compiler will accept with addr
being "(void *)(xen_pv_domain() ? xen_foo : foo)". :-)

>> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
>> index 01fd0a7f48cd..e2ae5f3b9c2c 100644
>> --- a/arch/x86/include/asm/traps.h
>> +++ b/arch/x86/include/asm/traps.h
>> @@ -13,9 +13,8 @@ asmlinkage void divide_error(void);
>> asmlinkage void debug(void);
>> asmlinkage void nmi(void);
>> asmlinkage void int3(void);
>> -asmlinkage void xen_debug(void);
>> -asmlinkage void xen_int3(void);
>> -asmlinkage void xen_stack_segment(void);
>> +asmlinkage void xendebug(void);
>> +asmlinkage void xenint3(void);
>
> What are xendebug and xenint3 for? Are they because they don't use IST on Xen?

I believe so.

>> __visible struct pv_cpu_ops pv_cpu_ops = {
>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index bf54309b85da..a79a97a46a59 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -946,15 +946,15 @@ void __init early_trap_init(void)
>> * early stage. DEBUG_STACK will be equipped after cpu_init() in
>> * trap_init().
>> *
>> - * We don't need to set trace_idt_table like set_intr_gate(),
>> + * We don't need to set trace_idt_table like set_intr_gate_pv(),
>> * since we don't have trace_debug and it will be reset to
>> * 'debug' in trap_init() by set_intr_gate_ist().
>> */
>> set_intr_gate_notrace(X86_TRAP_DB, debug);
>
> This is confusing IMO. Maybe just drop the comment change? But how
> does this work at all on Xen?

TBH: I don't whether it works on Xen. The changes I did here were just
mechanical ones.

>> - set_intr_gate(X86_TRAP_OLD_MF, coprocessor_segment_overrun);
>> - set_intr_gate(X86_TRAP_TS, invalid_TSS);
>> - set_intr_gate(X86_TRAP_NP, segment_not_present);
>> - set_intr_gate(X86_TRAP_SS, stack_segment);
>> - set_intr_gate(X86_TRAP_GP, general_protection);
>> - set_intr_gate(X86_TRAP_SPURIOUS, spurious_interrupt_bug);
>> - set_intr_gate(X86_TRAP_MF, coprocessor_error);
>> - set_intr_gate(X86_TRAP_AC, alignment_check);
>> + set_intr_gate_pv(X86_TRAP_OLD_MF, coprocessor_segment_overrun);
>> + set_intr_gate_pv(X86_TRAP_TS, invalid_TSS);
>> + set_intr_gate_pv(X86_TRAP_NP, segment_not_present);
>> + set_intr_gate_pv(X86_TRAP_SS, stack_segment);
>> + set_intr_gate_pv(X86_TRAP_GP, general_protection);
>> + set_intr_gate_pv(X86_TRAP_SPURIOUS, spurious_interrupt_bug);
>> + set_intr_gate_pv(X86_TRAP_MF, coprocessor_error);
>> + set_intr_gate_pv(X86_TRAP_AC, alignment_check);
>
> Hmm. I'm okay with this, but I'm wondering whether it might be nice
> to try to have a pv op that changes what IDT writes do and remaps the
> function pointer. Like...
>
>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>> index 811e4ddb3f37..7e107142bc4f 100644
>> --- a/arch/x86/xen/enlighten_pv.c
>> +++ b/arch/x86/xen/enlighten_pv.c
>> @@ -598,24 +598,22 @@ static int cvt_gate_to_trap(int vector, const gate_desc *val,
>> * 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) {
>> + if (addr == (unsigned long)xen_debug)
>> + addr = (unsigned long)xen_xendebug;
>> + else if (addr == (unsigned long)xen_int3)
>> + addr = (unsigned long)xen_xenint3;
>> + else if (addr == (unsigned long)xen_double_fault) {
>> /* Don't need to handle these */
>> return 0;
>
> ...this? Can't this list just be extended to handle all of the pv
> entries instead of using the macro magic?

I'll have a try.

>
>> diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
>> index c3df43141e70..f72ff71cc897 100644
>> --- a/arch/x86/xen/xen-asm_64.S
>> +++ b/arch/x86/xen/xen-asm_64.S
>> @@ -22,11 +22,46 @@
>>
>> #include "xen-asm.h"
>>
>> -ENTRY(xen_adjust_exception_frame)
>> - mov 8+0(%rsp), %rcx
>> - mov 8+8(%rsp), %r11
>> - ret $16
>> -ENDPROC(xen_adjust_exception_frame)
>> +.macro xen_pv_trap name
>> +ENTRY(xen_\name)
>> + pop %rcx
>> + pop %r11
>> + jmp \name
>> +END(xen_\name)
>> +.endm
>> +
>> +xen_pv_trap divide_error
>> +xen_pv_trap debug
>> +xen_pv_trap xendebug
>> +xen_pv_trap int3
>> +xen_pv_trap xenint3
>> +xen_pv_trap nmi
>> +xen_pv_trap overflow
>> +xen_pv_trap bounds
>> +xen_pv_trap invalid_op
>> +xen_pv_trap device_not_available
>> +xen_pv_trap double_fault
>> +xen_pv_trap coprocessor_segment_overrun
>> +xen_pv_trap invalid_TSS
>> +xen_pv_trap segment_not_present
>> +xen_pv_trap stack_segment
>> +xen_pv_trap general_protection
>> +xen_pv_trap page_fault
>> +xen_pv_trap async_page_fault
>> +xen_pv_trap spurious_interrupt_bug
>> +xen_pv_trap coprocessor_error
>> +xen_pv_trap alignment_check
>> +#ifdef CONFIG_X86_MCE
>> +xen_pv_trap machine_check
>> +#endif /* CONFIG_X86_MCE */
>> +xen_pv_trap simd_coprocessor_error
>> +#ifdef CONFIG_IA32_EMULATION
>> +xen_pv_trap entry_INT80_compat
>> +#endif
>> +#ifdef CONFIG_TRACING
>> +xen_pv_trap trace_page_fault
>> +#endif
>> +xen_pv_trap hypervisor_callback
>
> I like this.
>
> Also, IMO it would be nice to fully finish the job. Remaining steps are:
>
> 1. Unsuck the SYSCALL entries on Xen PV.
> 2. Unsuck the SYENTER entry on Xen PV.
> 3. Make a xen_nmi that's actually correct (should be trivial)
>
> #1 is here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry_syscall&id=14fee348e3e86c994400d68085217d1232a637d6
>
> Can you test it and, if you like it, either add it to your series or
> ack it? If you have extra spare cycles, you could try to do #2 and
> #3, too :) For #2, it might actually make sense to rig up the Xen
> sysenter entry to redirect to entry_SYSCALL_compat or even an entirely
> new entry point -- SYSENTER is really weird.

I'll look into it.

Juergen

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In reply to: Juergen Gross: "Re: [Xen-devel] [PATCH v2] xen: get rid of paravirt op adjust_exception_frame"
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