Re: [PATCH 2/2] arm64: kvm: upgrade csselr and ccsidr to 64-bit values

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Jan 24, 2017 at 10:55:24AM +0000, Sudeep Holla wrote:
> 
> 
> On 24/01/17 10:30, Christoffer Dall wrote:
> > On Tue, Jan 24, 2017 at 10:15:38AM +0000, Sudeep Holla wrote:
> >>
> >>
> >> On 23/01/17 21:08, Christoffer Dall wrote:
> >>> On Fri, Jan 20, 2017 at 10:50:10AM +0000, Sudeep Holla wrote:
> >>>> csselr and ccsidr are treated as 64-bit values already elsewhere in the
> >>>> kernel. It also aligns well with the architecture extensions that allow
> >>>> 64-bit format for ccsidr.
> >>>>
> >>>> This patch upgrades the existing accesses to csselr and ccsidr from
> >>>> 32-bit to 64-bit in preparation to add support to those extensions.
> >>>>
> >>>> Cc: Christoffer Dall <christoffer.dall@linaro.org>
> >>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
> >>>> Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> >>>> ---
> >>>>  arch/arm64/kvm/sys_regs.c | 18 +++++++++---------
> >>>>  1 file changed, 9 insertions(+), 9 deletions(-)
> >>>>
> >>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> >>>> index 5dca1f10340f..a3559a8a2b0c 100644
> >>>> --- a/arch/arm64/kvm/sys_regs.c
> >>>> +++ b/arch/arm64/kvm/sys_regs.c
> >>
> >> [..]
> >>
> >>>> @@ -2004,8 +2004,8 @@ static int demux_c15_get(u64 id, void __user *uaddr)
> >>>>  
> >>>>  static int demux_c15_set(u64 id, void __user *uaddr)
> >>>>  {
> >>>> -	u32 val, newval;
> >>>> -	u32 __user *uval = uaddr;
> >>>> +	u64 val, newval;
> >>>> +	u64 __user *uval = uaddr;
> >>>
> >>> Doesn't converting these uval pointers to u64 cause us to break the ABI
> >>> as we'll now be reading/writing 64-bit values to userspace with the
> >>> get_user and put_user following the declarations?
> >>>
> >>
> >> Yes, I too have similar concern. IIUC it is always read via kvm_one_reg
> >> structure. I could not find any specific user for this register to cross
> >> check.
> >>
> > 
> > Not sure it matters which interface we get the userspace pointer from?
> > 
> 
> Agreed.
> 
> > This patch is definitely changing the write from a 32-bit write to a
> > 64-bit write and there's a specific check prior to the put_user() call
> > which checks that userspace intended a 32-bit value and presumably
> > provided a 32-bit pointer.
> > 
> 
> I see you point, I missed to see that check(just to be sure KVM_REG_SIZE
> check right ?).

yes.


> 
> > So I think the only way to return 64-bit AArch32 system register values
> > to userspace (if that is the intention) is to define a new ID for 64-bit
> > CCSIDR registers and handle them separately.
> > 
> 
> I will add KVM_REG_ARM_DEMUX_ID_CCSIDR_64B or something similar.
> Thanks for the review.
> 

Cool, thanks.
-Christoffer