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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.


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