Re: [PATCH] x86/traps: Re-enable IRQs after reading cr2 in the #PF handler

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 11, 2024 at 03:58:23PM +0100, Alejandro Vallejo wrote:
> Moves sti directly after the cr2 read and immediately after the #PF
> handler.
> 
> While in the area, remove redundant q suffix to a movq in entry.S
> 
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> ---
> I don't think this is a bug as much as an accident about to happen. Even if
> there's no cases at the moment in which the IRQ handler may page fault, that
> might change in the future.
> 
> Note: I haven't tested it extensively beyond running it on GitLab.
> 
> pipeline:
>     https://gitlab.com/xen-project/people/agvallejo/xen/-/pipelines/1449182525
> 
> ---
>  xen/arch/x86/traps.c        |  2 ++
>  xen/arch/x86/x86_64/entry.S | 11 +++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
> index 708136f625..1c04c03d9f 100644
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -1600,6 +1600,8 @@ void asmlinkage do_page_fault(struct cpu_user_regs 
> *regs)
>  
>      addr = read_cr2();
>  
> +    local_irq_enable();

I would maybe add an ASSERT(!local_irq_is_enabled()); at the top of the
function, just to make sure the context is as expected.

> +
>      /* fixup_page_fault() might change regs->error_code, so cache it here. */
>      error_code = regs->error_code;
>  
> diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
> index b8482de8ee..ef803f6288 100644
> --- a/xen/arch/x86/x86_64/entry.S
> +++ b/xen/arch/x86/x86_64/entry.S
> @@ -844,8 +844,7 @@ handle_exception_saved:
>  #elif !defined(CONFIG_PV)
>          ASSERT_CONTEXT_IS_XEN
>  #endif /* CONFIG_PV */
> -        sti
> -1:      movq  %rsp,%rdi
> +1:      mov   %rsp,%rdi

Since you are modifying this already - we usually add a space between
the comma and the next operand.

Thanks, Roger.