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SubjectRe: [PATCH] x86/entry/64/paravirt: Use paravirt-safe macro to access eflags
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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@oracle.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) ?


Juergen

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