Re: [PATCH v2 3/7] KVM: arm: vgic-new: improve compatibility with 32-bit

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

On Tue, Sep 06, 2016 at 02:54:01PM +0100, Vladimir Murzin wrote:
> On 06/09/16 14:22, Christoffer Dall wrote:
> > On Tue, Sep 06, 2016 at 01:41:37PM +0100, Vladimir Murzin wrote:
> >> Hi Christoffer,
> >>
> >> On 05/09/16 12:29, Christoffer Dall wrote:
> >>> Hi Vladimir,
> >>>
> >>> I think commit title is too vague, can you be more specific?
> >>>
> >>
> >> KVM: arm: vgic-new: make extract_bytes to always work on 64-bit data
> >>
> >> is it better?
> > 
> > I would suggest:
> > 
> > KVM: arm: vgic: Support 64-bit data manipulation on 32-bit host systems
> > 
> >>
> >>> On Tue, Aug 16, 2016 at 11:46:54AM +0100, Vladimir Murzin wrote:
> >>>> We have couple of 64-bit register defined in GICv3 architecture, so
> >>>
> >>> 'a couple',  'registers' (plural)
> >>>
> >>>> "unsigned long" kind of accessors wouldn't work for 32-bit. However,
> >>>
> >>> 'wouldn't work for 32-bit' is kind of generic as well.  Perhaps you mean
> >>> that unsigned long accesses to these registers will only access a single
> >>> 32-bit work of that register.
> >>>
> >>>> these registers can't be access as 64-bit in a one go if we run 32-bit
> >>>
> >>> 'accessed', 's/in one go/with a single instruction/' ?
> >>>
> >>> 'a 32-bit host'
> >>>
> >>>> host simply because KVM doesn't support multiple load/store on MMIO
> >>>
> >>> by 'multiple load/store' you mean the 'load/store multiple' instructions
> >>> specifically, right?  Not a sequence of multiple loads and stores.  I
> >>> think you should be more specific here as well, for example, I think
> >>> ldrd and strd are problematic as well.
> >>>
> >>>> space.
> >>>>
> >>>> It means that 32-bit guest access these registers in 32-bit chunks, so
> >>>
> >>> 'a 32-bit guest', 'accesses'
> >>>
> >>
> >> all suggestions you've made above are true. I'll rework commit message
> >> to be more precise.
> >>
> > 
> > Thanks!
> > 
> >>>> the only thing we need to do is to ensure that extract_bytes() always
> >>>> takes 64-bit data.
> >>>>
> >>>> Since we are here fix couple of other width related issues by using
> >>>> ULL variants over UL.
> >>>>
> >>>> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
> >>>> ---
> >>>>  virt/kvm/arm/vgic/vgic-mmio-v3.c |    6 +++---
> >>>>  virt/kvm/arm/vgic/vgic-mmio.h    |    2 +-
> >>>>  2 files changed, 4 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >>>> index ff668e0..cc20b60 100644
> >>>> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >>>> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> >>>> @@ -23,7 +23,7 @@
> >>>>  #include "vgic-mmio.h"
> >>>>  
> >>>>  /* extract @num bytes at @offset bytes offset in data */
> >>>> -unsigned long extract_bytes(unsigned long data, unsigned int offset,
> >>>> +unsigned long extract_bytes(u64 data, unsigned int offset,
> >>>>  			    unsigned int num)
> >>>>  {
> >>>>  	return (data >> (offset * 8)) & GENMASK_ULL(num * 8 - 1, 0);
> >>>> @@ -179,7 +179,7 @@ static unsigned long vgic_mmio_read_v3r_typer(struct kvm_vcpu *vcpu,
> >>>>  	int target_vcpu_id = vcpu->vcpu_id;
> >>>>  	u64 value;
> >>>>  
> >>>> -	value = (mpidr & GENMASK(23, 0)) << 32;
> >>>> +	value = (mpidr & GENMASK_ULL(23, 0)) << 32;
> >>>
> >>> why does this make a difference when mpidr is an unsigned long?
> >>
> >> because we access a little bit further than unsigned long can accommodate
> >>
> >>   CC      arch/arm/kvm/../../../virt/kvm/arm/vgic/vgic-mmio-v3.o
> >> arch/arm/kvm/../../../virt/kvm/arm/vgic/vgic-mmio-v3.c: In function
> >> 'vgic_mmio_read_v3r_typer':
> >> arch/arm/kvm/../../../virt/kvm/arm/vgic/vgic-mmio-v3.c:184:35: warning:
> >> left shift count >= width of type [-Wshift-count-overflow]
> >>   value = (mpidr & GENMASK(23, 0)) << 32;
> >>                                    ^
> >>
> >> I can include this warning in commit message or maybe you want a
> >> separate patch?
> >>
> > My point was that the code doesn't really make sense when compiled on a
> > 32-bit platform without also modifing the type for the mpidr variable.
> > Am I missing something?
> 
> I've not seen any difference in generated code, but for consistency I'll
> update mpidr variable to u64.
> 

That could be because you need to update kvm_vcpu_get_mpidr_aff() to
return a u64 as well.

-Christoffer