Re: [Xen-devel] [PATCH 1/6] xen: extend XEN_DOMCTL_memory_mapping to handle cacheability

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 4/20/19 1:02 AM, Stefano Stabellini wrote:
> On Tue, 26 Feb 2019, Julien Grall wrote:
> > Hi,
> >
> > On 26/02/2019 23:07, Stefano Stabellini wrote:
> > > Reuse the existing padding field to pass cacheability information about
> > > the memory mapping, specifically, whether the memory should be mapped as
> > > normal memory or as device memory (this is what we have today).
> > >
> > > Add a cacheability parameter to map_mmio_regions. 0 means device
> > > memory, which is what we have today.
> > >
> > > On ARM, map device memory as p2m_mmio_direct_dev (as it is already done
> > > today) and normal memory as p2m_ram_rw.
> > >
> > > On x86, return error if the cacheability requested is not device memory.
> > >
> > > Signed-off-by: Stefano Stabellini <stefanos@xxxxxxxxxx>
> > > CC: JBeulich@xxxxxxxx
> > > CC: andrew.cooper3@xxxxxxxxxx
> > > ---
> > >    xen/arch/arm/gic-v2.c            |  3 ++-
> > >    xen/arch/arm/p2m.c               | 19 +++++++++++++++++--
> > >    xen/arch/arm/platforms/exynos5.c |  4 ++--
> > >    xen/arch/arm/platforms/omap5.c   |  8 ++++----
> > >    xen/arch/arm/vgic-v2.c           |  2 +-
> > >    xen/arch/arm/vgic/vgic-v2.c      |  2 +-
> > >    xen/arch/x86/hvm/dom0_build.c    |  7 +++++--
> > >    xen/arch/x86/mm/p2m.c            |  6 +++++-
> > >    xen/common/domctl.c              |  8 +++++---
> > >    xen/drivers/vpci/header.c        |  3 ++-
> > >    xen/include/public/domctl.h      |  4 +++-
> > >    xen/include/xen/p2m-common.h     |  3 ++-
> > >    12 files changed, 49 insertions(+), 20 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/gic-v2.c b/xen/arch/arm/gic-v2.c
> > > index e7eb01f..1ea3da2 100644
> > > --- a/xen/arch/arm/gic-v2.c
> > > +++ b/xen/arch/arm/gic-v2.c
> > > @@ -690,7 +690,8 @@ static int gicv2_map_hwdown_extra_mappings(struct domain *d)
> > >    
> > >            ret = map_mmio_regions(d, gaddr_to_gfn(v2m_data->addr),
> > >                                   PFN_UP(v2m_data->size),
> > > -                               maddr_to_mfn(v2m_data->addr));
> > > +                               maddr_to_mfn(v2m_data->addr),
> > > +                               CACHEABILITY_DEVMEM);
> > >            if ( ret )
> > >            {
> > >                printk(XENLOG_ERR "GICv2: Map v2m frame to d%d failed.\n",
> > > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > > index 30cfb01..5b8fcc5 100644
> > > --- a/xen/arch/arm/p2m.c
> > > +++ b/xen/arch/arm/p2m.c
> > > @@ -1068,9 +1068,24 @@ int unmap_regions_p2mt(struct domain *d,
> > >    int map_mmio_regions(struct domain *d,
> > >                         gfn_t start_gfn,
> > >                         unsigned long nr,
> > > -                     mfn_t mfn)
> > > +                     mfn_t mfn,
> > > +                     uint32_t cache_policy)
> > >    {
> > > -    return p2m_insert_mapping(d, start_gfn, nr, mfn, p2m_mmio_direct_dev);
> > > +    p2m_type_t t;
> > > +
> > > +    switch ( cache_policy )
> > > +    {
> > > +    case CACHEABILITY_MEMORY:
> > > +        t = p2m_ram_rw;
> > Potentially, you want to clean the cache here.
> We have been talking about this and I have been looking through the
> code. I am still not exactly sure how to proceed.
>
> Is there a reason why cacheable reserved_memory pages should be treated
> differently from normal memory, in regards to cleaning the cache? It
> seems to me that they should be the same in terms of cache issues?
Your wording is a bit confusing. I guess what you call "normal memory" 
is guest memory, am I right?
Any memory assigned to the guest is and clean & invalidate (technically 
clean is enough) before getting assigned to the guest (see 
flush_page_to_ram). So this patch is introducing a different behavior 
that what we currently have for other normal memory.
But my concern is you may inconsistently use the memory attributes 
breaking coherency. For instance, you map in Guest A with cacheable 
attributes then after the guest died, you remap to guest B with a 
non-cacheable attributes. guest B may have an inconsistent view of the 
memory mapped.
This is one case where cleaning the cache would be necessary. One could 
consider this is part of the "device reset" (this is a bit of the name 
abuse), so Xen should not take care of it.
The most important bit is to have documentation that reflect the issues 
with such parameters. So the user is aware of what could go wrong when 
using "iomem".
> Is there a place where we clean the dcache for normal pages, one that is
> not tied to p2m->clean_pte, which is different (it is there for iommu
> reasons)?
p2m->clean_pte is only here to deal with non-coherence IOMMU page-table 
walker. See above for flushing normal pages.
Cheers,

--
Julien Grall

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