linux-kernel - Re: [PATCH v9 01/26] arm64: Fix HCR.TGE status for NMI contexts

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Message-ID: <7a2c92ca-f0eb-1391-4dc4-005e1dbadc30@arm.com>
Date:   Thu, 31 Jan 2019 09:40:02 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Christoffer Dall <christoffer.dall@....com>
Cc:     James Morse <james.morse@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        daniel.thompson@...aro.org, joel@...lfernandes.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        mark.rutland@....com, Arnd Bergmann <arnd@...db.de>,
        linux-arch@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v9 01/26] arm64: Fix HCR.TGE status for NMI contexts



On 31/01/2019 09:27, Christoffer Dall wrote:
> On Thu, Jan 31, 2019 at 08:56:04AM +0000, Julien Thierry wrote:
>>
>>
>> On 31/01/2019 08:19, Christoffer Dall wrote:
>>> On Mon, Jan 28, 2019 at 03:42:42PM +0000, Julien Thierry wrote:
>>>> Hi James,
>>>>
>>>> On 28/01/2019 11:48, James Morse wrote:
>>>>> Hi Julien,
>>>>>
>>>>> On 21/01/2019 15:33, Julien Thierry wrote:
>>>>>> When using VHE, the host needs to clear HCR_EL2.TGE bit in order
>>>>>> to interract with guest TLBs, switching from EL2&0 translation regime
>>>>>
>>>>> (interact)
>>>>>
>>>>>
>>>>>> to EL1&0.
>>>>>>
>>>>>> However, some non-maskable asynchronous event could happen while TGE is
>>>>>> cleared like SDEI. Because of this address translation operations
>>>>>> relying on EL2&0 translation regime could fail (tlb invalidation,
>>>>>> userspace access, ...).
>>>>>>
>>>>>> Fix this by properly setting HCR_EL2.TGE when entering NMI context and
>>>>>> clear it if necessary when returning to the interrupted context.
>>>>>
>>>>> Yes please. This would not have been fun to debug!
>>>>>
>>>>> Reviewed-by: James Morse <james.morse@....com>
>>>>>
>>>>>
>>>>
>>>> Thanks.
>>>>
>>>>>
>>>>> I was looking for why we need core code to do this, instead of updating the
>>>>> arch's call sites. Your 'irqdesc: Add domain handlers for NMIs' patch (pointed
>>>>> to from the cover letter) is the reason: core-code calls nmi_enter()/nmi_exit()
>>>>> itself.
>>>>>
>>>>
>>>> Yes, that's the main reason.
>>>>
>>> I wondered the same thing, but I don't understand the explanation :(
>>>
>>> Why can't we do a local_daif_mask() around the (very small) calls that
>>> clear TGE instead?
>>>
>>
>> That would protect against the pseudo-NMIs, but you can still get an
>> SDEI at that point even with all daif bits set. Or did I misunderstand
>> how SDEI works?
>>
> 
> I don't know the details of SDEI.  From looking at this patch, the
> logical conclusion would be that SDEIs can then only be delivered once
> we've called nmi_enter, but since we don't call this directly from the
> code that clears TGE for doing guest TLB invalidation (or do we?) then
> masking interrupts at the PSTATE level should be sufficient.
> 
> Surely I'm missing some part of the bigger picture here.
> 

I'm not sure I understand. SDEI uses the NMI context and AFAIU, it is an
interrupt that the firmware sends to the OS, and it is sent regardless
of the PSTATE at the OS EL.

So, the worrying part is:
- Hyp clears TGE
- Exception/interrupt taken to EL3
- Firmware decides it's a good time to send an SDEI to the OS
- SDEI handler (at EL2 for VHE) does nmi_enter()
- SDEI handler needs to do cache invalidation or something with the
EL2&0 translation regime but TGE is cleared

We don't expect the code that clears TGE to call nmi_enter().

-- 
Julien Thierry