Re: [Xen-devel] [PATCH V5 05/10] xen/arm64: gicv3: Use AFF1 when translating ICC_SGI1R_EL1 to cpumask

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 31/05/2015 16:25, Chen Baozi wrote:
> > On May 31, 2015, at 21:14, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
> > On 30/05/2015 12:07, Chen Baozi wrote:
> > > From: Chen Baozi <baozich@xxxxxxxxx>
> > >
> > > To support more than 16 vCPUs, we have to calculate cpumask with AFF1
> > > field value in ICC_SGI1R_EL1.
> > >
> > > Signed-off-by: Chen Baozi <baozich@xxxxxxxxx>
> > > ---
> > >   xen/arch/arm/vgic-v3.c            | 9 ++++++++-
> > >   xen/include/asm-arm/gic_v3_defs.h | 3 +++
> > >   2 files changed, 11 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/arm/vgic-v3.c b/xen/arch/arm/vgic-v3.c
> > > index a283c8c..21d8d3f 100644
> > > --- a/xen/arch/arm/vgic-v3.c
> > > +++ b/xen/arch/arm/vgic-v3.c
> > > @@ -976,10 +976,17 @@ static inline void gicv3_sgir_to_cpumask(cpumask_t 
> > > *cpumask,
> > >                                            const register_t sgir)
> > >   {
> > >       unsigned long target_list;
> > > +    int aff1;
> > unsigned int.
> >
> > >       target_list = sgir & ICH_SGI_TARGETLIST_MASK;
> > > -    bitmap_copy(cpumask_bits(cpumask), &target_list, ICH_SGI_TARGET_BITS);
> > > +    /* We assume that only AFF1 is used in ICC_SGI1R_EL1. */
> > > +    aff1 = (sgir >> ICH_SGI_AFFINITY_LEVEL(1)) & ICH_SGI_AFFx_MASK;
> > >
> > > +    BUILD_BUG_ON(sizeof(cpumask_t)*8 < MAX_VIRT_CPUS);
> > Ah, here is the BUILD_BUG_ON. This is not vgic-v3 specific but generic to all 
> > the vgic. It would have been more logical to put it in the function vgic_to_sgi 
> > in the previous patch (i.e #4).
> >
> > > +    BUG_ON(((aff1+1) * ICH_SGI_TARGET_BITS) > NR_CPUS);
> > NACK. This value is passed by the guest. With this a malicious guest could take 
> > down Xen.
> >
> > > +
> > > +    memcpy((uint16_t *)cpumask + aff1, &target_list,
> > That's hackhish. You can't assume that the bitmap will be at the beginning of 
> > cpumask_t.
> Sorry, I’m not quite understand this. Why can not?
cpumask_t is a structure containing a bitmap. You are assuming that the 
address of cpumask_t is always equal to cpumask_t.bitmap.
There is helper to get the correct address (see cpumask_bits(cpumask)).

Furthermore, there is function to copy a bitmap (see bitmap_copy). 
Although it will require more than 1 line of code.
If you don't want to use it please use a direct assignation (it's only a 
16 bits data) which will always be faster than a memcpy.
Regards,

--
Julien Grall

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