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

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Message-ID: <20190131081900.GM13482@e113682-lin.lund.arm.com>
Date:   Thu, 31 Jan 2019 09:19:00 +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 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?


Thanks,

    Christoffer