linux-kernel - Re: Xen PV breakage after IRQ stack code refactoring

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Date:   Mon, 27 Nov 2017 08:30:51 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, Andy Lutomirski <luto@...nel.org>
Cc:     xen-devel <xen-devel@...ts.xen.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Xen PV breakage after IRQ stack code refactoring

On 11/27/2017 12:34 AM, Juergen Gross wrote:
> On 27/11/17 05:03, Andy Lutomirski wrote:
>> On Sun, Nov 26, 2017 at 9:10 AM, Boris Ostrovsky
>> <boris.ostrovsky@...cle.com> wrote:
>>> Andy,
>>>
>>> (Can't find the original patch in my mailbox)
>>>
>>> This hunk from 1d3e53e8624a ("x86/entry/64: Refactor IRQ stacks and make
>>> them NMI-safe")
>>>
>>>
>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>> index a9a8027..0d4483a 100644
>>> --- a/arch/x86/entry/entry_64.S
>>> +++ b/arch/x86/entry/entry_64.S
>>> @@ -447,6 +447,59 @@ ENTRY(irq_entries_start)
>>>      .endr
>>>  END(irq_entries_start)
>>>
>>> +.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
>>> +#ifdef CONFIG_DEBUG_ENTRY
>>> +       pushfq
>>> +       testl $X86_EFLAGS_IF, (%rsp)
>>> +       jz .Lokay_\@
>>> +       ud2
>>> +.Lokay_\@:
>>> +       addq $8, %rsp
>>> +#endif
>>> +.endm
>>> +
>>>
>>> makes Xen PV guests somewhat unhappy because IF flag will be set.
>>>
>>> I was hoping to use ALTERNATIVE instruction but when we hit this for the
>>> first time we haven't rewritten instructions yet. Moving check_bugs() a bit
>>> higher helps but because this is common code I don't know how well it will
>>> work on other architectures (and, in fact, whether it is even safe on x86 in
>>> general, although that can be verified).
>>>
>>> Another option is to also add a parameter to DEBUG_ENTRY_ASSERT_IRQS_OFF (or
>>> to ENTER_IRQ_STACK) from xen_do_hypervisor_callback (which is where the
>>> failure happens) but this looks pretty fragile in that it assumes that
>>> xen_do_hypervisor_callback is the only place where we use this codepath
>>> before alt instructions are set.
>>>
>>> Any other suggestions?
>> Do we have a convenient asm way to access the save_fl pvop?
> No, but adding it would be pretty straight forward. Its something like:
>
> #define SAVE_FLAGS(clobbers)                                        \
>         PARA_SITE(PARA_PATCH(pv_irq_ops, PV_IRQ_save_fl), clobbers, \
>                   PV_SAVE_REGS(clobbers | CLBR_CALLEE_SAVE);        \
>                   call PARA_INDIRECT(pv_irq_ops+PV_IRQ_save_fl);    \
>                   PV_RESTORE_REGS(clobbers | CLBR_CALLEE_SAVE);)
>
> similar to DISABLE_INTERRUPTS(), requiring just the definition of
> PV_IRQ_save_fl in asm-offsets.c and the non-pvops definition of
> SAVE_FLAGS() in irqflags.h.

Hmm.. Indeed, I haven't thought of this for whatever reasons. I'll send
the patch soon.

Thanks.
-boris