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

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Date:   Thu, 31 Jan 2019 10:48:53 +0100
From:   Christoffer Dall <christoffer.dall@....com>
To:     Julien Thierry <julien.thierry@....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 Thu, Jan 31, 2019 at 09:40:02AM +0000, Julien Thierry wrote:
> 
> 
> 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().
> 

You do understand :)

I didn't understand that the SDEI handler calls nmi_enter() -- and to be
fair the commit message didn't really provide that link -- but it
makes perfect sense now.  I naively thought that SDEI had respected the
pstate bits setting before, and that this was becoming a problem with
the introduction of pseudo-NMIs, but I clearly came at this from the
wrong direction.


Thanks for the explanation!

    Christoffer