Re: [PATCH qemu] vfio/spapr: Fix page size calculation

[Greg Kurz <groug@kaod.org>]

On Tue, 24 Mar 2020 14:27:35 +0100
Philippe Mathieu-Daudé <philmd@redhat.com> wrote:

> 
> 
> On 3/24/20 7:39 AM, Alexey Kardashevskiy wrote:
> > Coverity detected an issue (CID 1421903) with potential call of clz64(0)
> > which returns 64 which make it do "<<" with a negative number.
> > 
> > This checks the mask and avoids undefined behaviour.
> > 
> > In practice pgsizes and memory_region_iommu_get_min_page_size() always
> > have some common page sizes and even if they did not, the resulting page
> > size would be 0x8000.0000.0000.0000 (gcc 9.2) and
> > ioctl(VFIO_IOMMU_SPAPR_TCE_CREATE) would fail anyway.
> > 
> > Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> > ---
> >   hw/vfio/spapr.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/vfio/spapr.c b/hw/vfio/spapr.c
> > index 33692fc86fd6..2900bd19417a 100644
> > --- a/hw/vfio/spapr.c
> > +++ b/hw/vfio/spapr.c
> > @@ -147,7 +147,7 @@ int vfio_spapr_create_window(VFIOContainer *container,
> >   {
> >       int ret = 0;
> >       IOMMUMemoryRegion *iommu_mr = IOMMU_MEMORY_REGION(section->mr);
> > -    uint64_t pagesize = memory_region_iommu_get_min_page_size(iommu_mr);
> > +    uint64_t pagesize = memory_region_iommu_get_min_page_size(iommu_mr), pgmask;
> >       unsigned entries, bits_total, bits_per_level, max_levels;
> >       struct vfio_iommu_spapr_tce_create create = { .argsz = sizeof(create) };
> >       long rampagesize = qemu_minrampagesize();
> > @@ -159,8 +159,8 @@ int vfio_spapr_create_window(VFIOContainer *container,
> >       if (pagesize > rampagesize) {
> >           pagesize = rampagesize;
> >       }
> > -    pagesize = 1ULL << (63 - clz64(container->pgsizes &
> > -                                   (pagesize | (pagesize - 1))));
> > +    pgmask = container->pgsizes & (pagesize | (pagesize - 1));
> 
> Is that ROUND_UP(container->pgsizes, pagesize)?
> 

This means we clip all page sizes greater than pagesize from
container->pgsizes... It doesn't look like ROUND_UP() semantics
to me.

> > +    pagesize = pgmask ? (1ULL << (63 - clz64(pgmask))) : 0;
> >       if (!pagesize) {
> >           error_report("Host doesn't support page size 0x%"PRIx64
> >                        ", the supported mask is 0x%lx",
> > 
> 
>