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

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Date:   Thu, 31 Jan 2019 08:56:04 +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 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?

Thanks,

-- 
Julien Thierry