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SubjectRe: [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@oracle.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@oracle.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@oracle.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
>>>
>>>
>

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