Re: [PATCH v2 1/2] s390x/pci: add support for guests that request direct mapping

[David Hildenbrand]

On 16.12.24 16:37, Philippe Mathieu-Daudé wrote:
> Hi Matthew,
>
> On 13/12/24 23:54, Matthew Rosato wrote:
> > When receiving a guest mpcifc(4) or mpcifc(6) instruction without the T
> > bit set, treat this as a request to perform direct mapping instead of
> > address translation.  In order to facilitate this, pin the entirety of
> > guest memory into the host iommu.
> >
> > Subsequent guest DMA operations are all expected to be of the format
> > guest_phys+sdma, allowing them to be used as lookup into the host
> > iommu table.
> >
> > Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > ---
> >    hw/s390x/s390-pci-bus.c         | 34 +++++++++++++++++++++++++++++++--
> >    hw/s390x/s390-pci-inst.c        | 13 +++++++++++--
> >    hw/s390x/s390-virtio-ccw.c      |  5 +++++
> >    include/hw/s390x/s390-pci-bus.h |  4 ++++
> >    4 files changed, 52 insertions(+), 4 deletions(-)
> >
> > diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
> > index 40b2567aa7..95dbe0c984 100644
> > --- a/hw/s390x/s390-pci-bus.c
> > +++ b/hw/s390x/s390-pci-bus.c
> > @@ -18,6 +18,7 @@
> >    #include "hw/s390x/s390-pci-inst.h"
> >    #include "hw/s390x/s390-pci-kvm.h"
> >    #include "hw/s390x/s390-pci-vfio.h"
> > +#include "hw/boards.h"
> >    #include "hw/pci/pci_bus.h"
> >    #include "hw/qdev-properties.h"
> >    #include "hw/pci/pci_bridge.h"
> > @@ -720,16 +721,43 @@ void s390_pci_iommu_enable(S390PCIIOMMU *iommu)
> >                                 TYPE_S390_IOMMU_MEMORY_REGION, 
> > OBJECT(&iommu->mr),
> >                                 name, iommu->pal + 1);
> >        iommu->enabled = true;
> > +    iommu->direct_map = false;
> >        memory_region_add_subregion(&iommu->mr, 0, 
> > MEMORY_REGION(&iommu->iommu_mr));
> >        g_free(name);
> >    }
> >    
> > +void s390_pci_iommu_dm_enable(S390PCIIOMMU *iommu)
> > +{
> > +    MachineState *ms = MACHINE(qdev_get_machine());
> > +
> > +    /*
> > +     * For direct-mapping we must map the entire guest address space.  Rather
> > +     * than using an iommu, create a memory region alias that maps GPA X to
> > +     * iova X + SDMA.  VFIO will handle pinning via its memory listener.
> > +     */
> > +    g_autofree char *name = g_strdup_printf("iommu-dm-s390-%04x",
> > +                                            iommu->pbdev->uid);
> > +    memory_region_init_alias(&iommu->dm_mr, OBJECT(&iommu->mr), name, ms->ram,
> > +                             0, ms->ram_size);
>
> Is it a good idea to take the whole machine ram-size here?
> Could it be better to pass it as qdev property?

I think we want all guest RAM, just like ordinary vfio on !s390x without 
a viommu would do.

Matthew, I assume to handle virtio-mem, we would actually pass in here 
the result from s390_get_memory_limit(), which will cover initial+device 
RAM, correct? Until then, this would map initial RAM only.

--
Cheers,

David / dhildenb