Re: [Xen-devel] [PATCH 5/9] xen: arm: Handle CP15 register traps from userspace

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2014-09-10 at 11:56 -0700, Julien Grall wrote:
> 
> On 10/09/14 02:48, Ian Campbell wrote:
> > On Tue, 2014-09-09 at 16:42 -0700, Julien Grall wrote:
> >> Hi Ian,
> >>
> >> On 09/09/14 09:23, Ian Campbell wrote:
> >>> Previously userspace access to PM* would have been incorrectly (but 
> >>> benignly)
> >>> implemented as RAZ/WI when running on a 32-bit kernel and would cause a
> >>> hypervisor exception (host crash) when running a 64-bit kernel (this was
> >>> already solved via the fix to XSA-102).
> >>>
> >>> CLIDR, CCSIDR, DCCISW, ACTLR, PMINTENSET, PMINTENCLR are EL1 only, 
> >>> attempts to
> >>> access from EL0 will trap to EL1 not to us, hence BUG_ON is appropriate 
> >>> now.
> >>
> >> In the unlikely case it happens, I don't think it will harm Xen, but
> >> only the guest. So is the BUG_ON really necessary on these registers?
> >>
> >> I think we should use BUG_ON when we know that it will harm the Xen and
> >> it's not possible to come back from the state.
> >
> > AIUI it would be a hardware bug to see these traps in Xen. I think a
> > BUG_ON is acceptable for such an occurrence.
> 
> I would turn into an ASSERT to avoid checking it in non-debug build.

BUG_ON is what we normally use in these circumstances. If we did hit
this case we would want to stop, not carry on with hardware in a
potentially unknown/unpredictable state, even for a non-debug build.

Ian.


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