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

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Date:   Mon, 27 Nov 2017 16:40:25 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, luto@...nel.org
Subject: Re: [PATCH] x86/entry/64/paravirt: Use paravirt-safe macro to access
 eflags

On 11/27/2017 02:22 PM, Juergen Gross wrote:
> On 27/11/17 19:05, Boris Ostrovsky 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>
>> ---
>>  arch/x86/entry/entry_64.S        |    5 ++---
>>  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, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index f81d50d..4bb7719 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -466,12 +466,11 @@ END(irq_entries_start)
>>  
>>  .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
>>  #ifdef CONFIG_DEBUG_ENTRY
>> -	pushfq
>> -	testl $X86_EFLAGS_IF, (%rsp)
>> +	SAVE_FLAGS(CLBR_ANY)
>> +	testl $X86_EFLAGS_IF, %eax
> Are you sure %eax is allowed to be modified?
>
>>  	jz .Lokay_\@
>>  	ud2
>>  .Lokay_\@:
>> -	addq $8, %rsp
>>  #endif
>>  .endm
>>  
>> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
>> index c8ef23f..7f65f3f 100644
>> --- a/arch/x86/include/asm/irqflags.h
>> +++ b/arch/x86/include/asm/irqflags.h
>> @@ -142,6 +142,9 @@ static inline notrace unsigned long arch_local_irq_save(void)
>>  	swapgs;					\
>>  	sysretl
>>  
>> +#ifdef CONFIG_DEBUG_ENTRY
>> +#define SAVE_FLAGS(x)		pushfq
> Isn't there a "pop %rax" missing (assuming %rax is allowed to be
> modified) ?


Yes to both. Need to save %rax in DEBUG_ENTRY_ASSERT_IRQS_OFF.

Thanks.
-boris