Re: [Xen-devel] [PATCH] mm: fix LLVM code-generation issue

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/11/2018 16:07, Roger Pau Monné wrote:
> On Thu, Nov 22, 2018 at 03:23:41PM +0000, Andrew Cooper wrote:
>> On 22/11/2018 15:20, Roger Pau Monné wrote:
>>> On Thu, Nov 22, 2018 at 02:03:55PM +0000, Julien Grall wrote:
>>>> Hi Jan,
>>>>
>>>> On 11/22/18 1:36 PM, Jan Beulich wrote:
>>>>>>>> On 22.11.18 at 14:31, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>> I think Julien's point is that without explicitly barriers, CPU0's
>>>>>> update to system_state may not be visible on CPU1, even though the
>>>>>> mappings have been shot down.
>>>>>>
>>>>>> Therefore, from the processors point of view, it did everything
>>>>>> correctly, and hit a real pagefault.
>>>>> Boot time updates of system_state should be of no interest here,
>>>>> as at that time the APs are all idling.
>>>> That's probably true today. But this code looks rather fragile as you don't
>>>> know how this is going to be used in the future.
>>>>
>>>> If you decide to gate init code with system_state, then you need a barrier
>>>> to ensure the code is future proof.
>>> Wouldn't it be enough to declare system_state as volatile?
>> No.  volatility (or lack thereof) is a compiler level construct.
>>
>> ARM has weaker cache coherency than x86, so a write which has completed
>> on one CPU0 in the past may legitimately not be visible on CPU1 yet.
>>
>> If you need guarantees about the visibility of updated, you must use
>> appropriate barriers.
> Right. There's some differences between ARM and x86, ARM sets
> SYS_STATE_active and continues to make use of init functions. In any
> case I have the following diff:
>
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index e83221ab79..cf50d05620 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -966,6 +966,7 @@ void __init start_xen(unsigned long boot_phys_offset,
>      serial_endboot();
>  
>      system_state = SYS_STATE_active;
> +    smp_wmb();
>  
>      create_domUs();
>  
> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> index 9cbff22fb3..41044c0b6f 100644
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -593,6 +593,7 @@ static void noinline init_done(void)
>      unsigned long start, end;
>  
>      system_state = SYS_STATE_active;
> +    smp_wmb();
>  
>      domain_unpause_by_systemcontroller(dom0);
>  
>

I'm afraid that that won't do anything to help at all.

smp_{wmb,rmb}() must be in matched pairs, and mb() must be matched with
itself.

~Andrew

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