Re: [Xen-devel] [PATCH V3] vmx/nmi: Do not use self_nmi() in VMEXIT handler

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/02/13 15:41, Jan Beulich wrote:
>>>> On 28.02.13 at 15:42, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> ... this must not be done when on the NMI stack (i.e. when the
>> NMI was raised while in hypervisor context). Checking for this
>> here would be strait forward, but I was really considering to do
>> all of this in the assembly exit path, and I was still undecided
>> whether we shouldn't follow Linux in skipping softirq processing
>> (and hence scheduling) on the way out from an NMI (I don't
>> think we'd need to do the same for MCE).
> Like this:
>
> x86: skip processing events on the NMI exit path
>
> Otherwise, we may end up in the scheduler, keeping NMIs masked for a
> possibly unbounded time (until whenever the next IRET gets executed).
>
> Of course it's open for discussion whether to always use the strait
> exit path from handle_ist_exception.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

The logic looks good to me.  As for the other IST exceptions, we will
never return from a double fault, and will rarely return from an MCE, so
I would say it doesn't really matter at the moment.

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

~Andrew

>
> --- a/xen/arch/x86/x86_64/compat/entry.S
> +++ b/xen/arch/x86/x86_64/compat/entry.S
> @@ -171,7 +171,7 @@ compat_bad_hypercall:
>          jmp  compat_test_all_events
>  
>  /* %rbx: struct vcpu, interrupts disabled */
> -compat_restore_all_guest:
> +ENTRY(compat_restore_all_guest)
>          ASSERT_INTERRUPTS_DISABLED
>          RESTORE_ALL adj=8 compat=1
>  .Lft0:  iretq
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -635,6 +635,9 @@ ENTRY(early_page_fault)
>          jmp   restore_all_xen
>          .popsection
>  
> +ENTRY(nmi)
> +        pushq $0
> +        movl  $TRAP_nmi,4(%rsp)
>  handle_ist_exception:
>          SAVE_ALL
>          testb $3,UREGS_cs(%rsp)
> @@ -649,12 +652,17 @@ handle_ist_exception:
>          movzbl UREGS_entry_vector(%rsp),%eax
>          leaq  exception_table(%rip),%rdx
>          callq *(%rdx,%rax,8)
> -        jmp   ret_from_intr
> +        cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
> +        jne   ret_from_intr
>  
> -ENTRY(nmi)
> -        pushq $0
> -        movl  $TRAP_nmi,4(%rsp)
> -        jmp   handle_ist_exception
> +        /* We want to get strait to the IRET in the NMI exit path. */
> +        testb $3,UREGS_cs(%rsp)
> +        GET_CURRENT(%rbx)
> +        jz    restore_all_xen
> +        movq  VCPU_domain(%rbx),%rax
> +        testb $1,DOMAIN_is_32bit_pv(%rax)
> +        jz    restore_all_guest
> +        jmp   compat_restore_all_guest
>  
>  ENTRY(nmi_crash)
>          pushq $0
>
>
>


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