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SubjectRe: [RFC 08/17] x86/asm/64: De-Xen-ify our NMI code
On Thu, Sep 7, 2017 at 2:34 AM, Juergen Gross <jgross@suse.com> wrote:
> On 06/09/17 23:36, Andy Lutomirski wrote:
>> Xen PV is fundamentally incompatible with our fancy NMI code: it
>> doesn't use IST at all, and Xen entries clobber two stack slots
>> below the hardware frame.
>>
>> Drop Xen PV support from our NMI code entirely.
>>
>> XXX: Juergen: could you write and test the tiny patch needed to
>> make Xen PV have a xen_nmi entry that handles NMIs? I don't know
>> how to test it.
>
> You mean something like the attached one?

Yes. Mind if I add it to my series?

>
> Seems to work at least for the "normal" case of a NMI coming in at
> a random point in time.
>
> Regarding testing: in case you have a Xen setup you can easily send
> a NMI to a domain from dom0:
>
> xl trigger <domain> nmi

Thanks!

>
>
> Juergen
>
>>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>> arch/x86/entry/entry_64.S | 41 +++++++++++++++++------------------------
>> 1 file changed, 17 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index a9e318f7cc9b..c81e05fb999e 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1171,21 +1171,12 @@ ENTRY(error_exit)
>> jmp retint_user
>> END(error_exit)
>>
>> -/* Runs on exception stack */
>> +/*
>> + * Runs on exception stack. Xen PV does not go through this path at all,
>> + * so we can use real assembly here.
>> + */
>> 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.
>> - */
>> - PARAVIRT_ADJUST_EXCEPTION_FRAME
>> -
>> - /*
>> * We allow breakpoints in NMIs. If a breakpoint occurs, then
>> * the iretq it performs will take us out of NMI context.
>> * This means that we can have nested NMIs where the next
>> @@ -1240,7 +1231,7 @@ ENTRY(nmi)
>> * stacks lest we corrupt the "NMI executing" variable.
>> */
>>
>> - SWAPGS_UNSAFE_STACK
>> + swapgs
>> cld
>> movq %rsp, %rdx
>> movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>> @@ -1402,7 +1393,7 @@ nested_nmi_out:
>> popq %rdx
>>
>> /* We are returning to kernel mode, so this cannot result in a fault. */
>> - INTERRUPT_RETURN
>> + iretq
>>
>> first_nmi:
>> /* Restore rdx. */
>> @@ -1432,7 +1423,7 @@ first_nmi:
>> pushfq /* RFLAGS */
>> pushq $__KERNEL_CS /* CS */
>> pushq $1f /* RIP */
>> - INTERRUPT_RETURN /* continues at repeat_nmi below */
>> + iretq /* continues at repeat_nmi below */
>> 1:
>> #endif
>>
>> @@ -1502,20 +1493,22 @@ nmi_restore:
>> /*
>> * Clear "NMI executing". Set DF first so that we can easily
>> * distinguish the remaining code between here and IRET from
>> - * the SYSCALL entry and exit paths. On a native kernel, we
>> - * could just inspect RIP, but, on paravirt kernels,
>> - * INTERRUPT_RETURN can translate into a jump into a
>> - * hypercall page.
>> + * the SYSCALL entry and exit paths.
>> + *
>> + * We arguably should just inspect RIP instead, but I (Andy) wrote
>> + * this code when I had the misapprehension that Xen PV supported
>> + * NMIs, and Xen PV would break that approach.
>> */
>> std
>> movq $0, 5*8(%rsp) /* clear "NMI executing" */
>>
>> /*
>> - * INTERRUPT_RETURN reads the "iret" frame and exits the NMI
>> - * stack in a single instruction. We are returning to kernel
>> - * mode, so this cannot result in a fault.
>> + * iretq reads the "iret" frame and exits the NMI stack in a
>> + * single instruction. We are returning to kernel mode, so this
>> + * cannot result in a fault. Similarly, we don't need to worry
>> + * about espfix64 on the way back to kernel mode.
>> */
>> - INTERRUPT_RETURN
>> + iretq
>> END(nmi)
>>
>> ENTRY(ignore_sysret)
>>
>

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