linux-kernel - Re: [PATCH v2] x86/entry/64/paravirt: Use paravirt-safe macro to access eflags

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Date:   Fri, 1 Dec 2017 09:44:20 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH v2] x86/entry/64/paravirt: Use paravirt-safe macro to
 access eflags

On Fri, Dec 1, 2017 at 8:57 AM, Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
> On 12/01/2017 11:22 AM, Andy Lutomirski wrote:
>> On Tue, Nov 28, 2017 at 7:28 AM, Boris Ostrovsky
>> <boris.ostrovsky@...cle.com> wrote:
>>> Commit 1d3e53e8624a ("x86/entry/64: Refactor IRQ stacks and make
>>> them NMI-safe") added DEBUG_ENTRY_ASSERT_IRQS_OFF macro that acceses
>>> eflags using 'pushfq' instruction when testing for IF bit. On PV Xen
>>> guests looking at IF flag directly will always see it set, resulting
>>> in 'ud2'.
>>>
>>> Introduce SAVE_FLAGS() macro that will use appropriate save_fl pv op
>>> when running paravirt.
>>>
>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> ---
>>> V2:
>>> * Preserve %rax in DEBUG_ENTRY_ASSERT_IRQS_OFF
>>> * Return (pop) %rax in SAVE_FLAGS for !CONFIG_PARAVIRT (irqflags.h)
>>>
>>>  arch/x86/entry/entry_64.S        |    7 ++++---
>>>  arch/x86/include/asm/irqflags.h  |    3 +++
>>>  arch/x86/include/asm/paravirt.h  |    9 +++++++++
>>>  arch/x86/kernel/asm-offsets_64.c |    3 +++
>>>  4 files changed, 19 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>> index f81d50d..c208dc1 100644
>>> --- a/arch/x86/entry/entry_64.S
>>> +++ b/arch/x86/entry/entry_64.S
>>> @@ -466,12 +466,13 @@ END(irq_entries_start)
>>>
>>>  .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
>>>  #ifdef CONFIG_DEBUG_ENTRY
>>> -       pushfq
>>> -       testl $X86_EFLAGS_IF, (%rsp)
>>> +       pushq %rax
>>> +       SAVE_FLAGS(CLBR_ANY)
>>> +       testl $X86_EFLAGS_IF, %eax
>> Confused.  You're both using CLBR_ANY and RAX.  Did you perhaps mean CLBR_NONE?
>
> CLBR_NONE will restore all registers, won't it? So it should be
> CLBR_RAX, should it? Otherwise we'll lose return value.

Ugh, probably right.  I've never grokked exactly what CLBR_ does.

>
> -boris
>
>>
>>>         jz .Lokay_\@
>>>         ud2
>>>  .Lokay_\@:
>>> -       addq $8, %rsp
>>> +       popq %rax
>>>  #endif
>>>  .endm
>>>
>>>
>