Re: [PATCH v3 05/10] vfio: Support for RamDiscardMgr in the !vIOMMU case

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 17 Dec 2020 19:55:55 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 17.12.20 19:36, Alex Williamson wrote:
> > On Wed, 16 Dec 2020 15:11:55 +0100
> > David Hildenbrand <david@redhat.com> wrote:
> >   
> >> Implement support for RamDiscardMgr, to prepare for virtio-mem
> >> support. Instead of mapping the whole memory section, we only map
> >> "populated" parts and update the mapping when notified about
> >> discarding/population of memory via the RamDiscardListener. Similarly, when
> >> syncing the dirty bitmaps, sync only the actually mapped (populated) parts
> >> by replaying via the notifier.
> >>
> >> Small mapping granularity is problematic for vfio, because we might run out
> >> of mappings. Indicate virito-mem as one of the problematic parts when
> >> warning in vfio_container_dma_reserve() to at least make users aware that
> >> there is such a limitation.
> >>
> >> Using virtio-mem with vfio is still blocked via
> >> ram_block_discard_disable()/ram_block_discard_require() after this patch.
> >>
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> >> Cc: Alex Williamson <alex.williamson@redhat.com>
> >> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >> Cc: Igor Mammedov <imammedo@redhat.com>
> >> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> >> Cc: Peter Xu <peterx@redhat.com>
> >> Cc: Auger Eric <eric.auger@redhat.com>
> >> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> >> Cc: teawater <teawaterz@linux.alibaba.com>
> >> Cc: Marek Kedzierski <mkedzier@redhat.com>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >>  hw/vfio/common.c              | 213 +++++++++++++++++++++++++++++++++-
> >>  include/hw/vfio/vfio-common.h |  13 +++
> >>  2 files changed, 225 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> >> index 5ad88d476f..b1582be1e8 100644
> >> --- a/hw/vfio/common.c
> >> +++ b/hw/vfio/common.c
> >> @@ -296,7 +296,8 @@ static void vfio_container_dma_reserve(VFIOContainer *container,
> >>      container->dma_reserved += dma_mappings;
> >>      if (!warned && container->dma_max &&
> >>          container->dma_reserved > container->dma_max) {
> >> -        warn_report("%s: possibly running out of DMA mappings. "
> >> +        warn_report("%s: possibly running out of DMA mappings. E.g., try"
> >> +                    " increasing the 'block-size' of virtio-mem devies."
> >>                      " Maximum number of DMA mappings: %d", __func__,
> >>                      container->dma_max);
> >>      }
> >> @@ -674,6 +675,146 @@ out:
> >>      rcu_read_unlock();
> >>  }
> >>  
> >> +static void vfio_ram_discard_notify_discard(RamDiscardListener *rdl,
> >> +                                            const MemoryRegion *mr,
> >> +                                            ram_addr_t offset, ram_addr_t size)
> >> +{
> >> +    VFIORamDiscardListener *vrdl = container_of(rdl, VFIORamDiscardListener,
> >> +                                                listener);
> >> +    const hwaddr mr_start = MAX(offset, vrdl->offset_within_region);
> >> +    const hwaddr mr_end = MIN(offset + size,
> >> +                              vrdl->offset_within_region + vrdl->size);
> >> +    const hwaddr iova = mr_start - vrdl->offset_within_region +
> >> +                        vrdl->offset_within_address_space;
> >> +    int ret;
> >> +
> >> +    if (mr_start >= mr_end) {
> >> +        return;
> >> +    }
> >> +
> >> +    /* Unmap with a single call. */
> >> +    ret = vfio_dma_unmap(vrdl->container, iova, mr_end - mr_start, NULL);
> >> +    if (ret) {
> >> +        error_report("%s: vfio_dma_unmap() failed: %s", __func__,
> >> +                     strerror(-ret));
> >> +    }
> >> +}
> >> +
> >> +static int vfio_ram_discard_notify_populate(RamDiscardListener *rdl,
> >> +                                            const MemoryRegion *mr,
> >> +                                            ram_addr_t offset, ram_addr_t size)
> >> +{
> >> +    VFIORamDiscardListener *vrdl = container_of(rdl, VFIORamDiscardListener,
> >> +                                                listener);
> >> +    const hwaddr mr_end = MIN(offset + size,
> >> +                              vrdl->offset_within_region + vrdl->size);
> >> +    hwaddr mr_start = MAX(offset, vrdl->offset_within_region);
> >> +    hwaddr mr_next, iova;
> >> +    void *vaddr;
> >> +    int ret;
> >> +
> >> +    /*
> >> +     * Map in (aligned within memory region) minimum granularity, so we can
> >> +     * unmap in minimum granularity later.
> >> +     */
> >> +    for (; mr_start < mr_end; mr_start = mr_next) {
> >> +        mr_next = QEMU_ALIGN_UP(mr_start + 1, vrdl->granularity);
> >> +        mr_next = MIN(mr_next, mr_end);
> >> +
> >> +        iova = mr_start - vrdl->offset_within_region +
> >> +               vrdl->offset_within_address_space;
> >> +        vaddr = memory_region_get_ram_ptr(vrdl->mr) + mr_start;
> >> +
> >> +        ret = vfio_dma_map(vrdl->container, iova, mr_next - mr_start,
> >> +                           vaddr, mr->readonly);
> >> +        if (ret) {
> >> +            /* Rollback */
> >> +            vfio_ram_discard_notify_discard(rdl, mr, offset, size);
> >> +            return ret;
> >> +        }
> >> +    }
> >> +    return 0;
> >> +}
> >> +
> >> +static void vfio_ram_discard_notify_discard_all(RamDiscardListener *rdl,
> >> +                                                const MemoryRegion *mr)
> >> +{
> >> +    VFIORamDiscardListener *vrdl = container_of(rdl, VFIORamDiscardListener,
> >> +                                                listener);
> >> +    int ret;
> >> +
> >> +    /* Unmap with a single call. */
> >> +    ret = vfio_dma_unmap(vrdl->container, vrdl->offset_within_address_space,
> >> +                         vrdl->size, NULL);
> >> +    if (ret) {
> >> +        error_report("%s: vfio_dma_unmap() failed: %s", __func__,
> >> +                     strerror(-ret));
> >> +    }
> >> +}
> >> +
> >> +static void vfio_register_ram_discard_notifier(VFIOContainer *container,
> >> +                                               MemoryRegionSection *section)
> >> +{
> >> +    RamDiscardMgr *rdm = memory_region_get_ram_discard_mgr(section->mr);
> >> +    RamDiscardMgrClass *rdmc = RAM_DISCARD_MGR_GET_CLASS(rdm);
> >> +    VFIORamDiscardListener *vrdl;
> >> +
> >> +    vrdl = g_new0(VFIORamDiscardListener, 1);
> >> +    vrdl->container = container;
> >> +    vrdl->mr = section->mr;
> >> +    vrdl->offset_within_region = section->offset_within_region;
> >> +    vrdl->offset_within_address_space = section->offset_within_address_space;
> >> +    vrdl->size = int128_get64(section->size);
> >> +    vrdl->granularity = rdmc->get_min_granularity(rdm, section->mr);
> >> +    vrdl->dma_max = vrdl->size / vrdl->granularity;
> >> +    if (!QEMU_IS_ALIGNED(vrdl->size, vrdl->granularity) ||
> >> +        !QEMU_IS_ALIGNED(vrdl->offset_within_region, vrdl->granularity)) {
> >> +        vrdl->dma_max++;
> >> +    }
> >> +
> >> +    /* Ignore some corner cases not relevant in practice. */
> >> +    g_assert(QEMU_IS_ALIGNED(vrdl->offset_within_region, TARGET_PAGE_SIZE));
> >> +    g_assert(QEMU_IS_ALIGNED(vrdl->offset_within_address_space,
> >> +                             TARGET_PAGE_SIZE));
> >> +    g_assert(QEMU_IS_ALIGNED(vrdl->size, TARGET_PAGE_SIZE));
> >> +
> >> +    /* We could consume quite some mappings later. */
> >> +    vfio_container_dma_reserve(container, vrdl->dma_max);  
> > 
> > 
> > Aha, I guess this is where the "reservation" aspect begins to appear.
> > Should this be its own counter though, perhaps
> > dma_discard_max_mappings?  The populate and discard callbacks could
> > further divide this into used and outstanding counters.  However, TBH
> > I'm not sure I understand the counters since this is probably the most
> > robust mapping path where we can actually safely nak a populate  
> 
> I'd like to be able to warn early on fundamental setup issues, not only
> when accidentally running into these limits later.
> 
> > callback.  Maybe rather than any of these runtime counters we should
> > just walk the vrdl_list, calculate max mappings, and if that exceeds
> > some large fraction of available mappings, issue a warning (not that
> > they wouldn't be useful for tracing).  Thanks,  
> 
> Sure, we can calculate max mappings from the vrdl_list. But which
> fraction to chose? The reservation approach simply considers any
> mappings (well, except IOMMU because they are kind of special)

Right, but we're looking at the address space of a device, which should
be exclusively system memory or an IOMMU range, right?  There are IOMMUs
that don't restrict the device to the IOVA window, but I'm not sure if
we care about those.  If that's true, I'm not sure we need to worry
about the complicated intersection of RamDiscardMgr and vIOMMU both
creating mappings.

> Guidance on the fraction / #mappings to assume we can use appreciated.

Can we use the number of KVM memory slots as a guide?  This is
essentially a mechanism for sub-dividing things that exist in a KVM
memory slot, so it seems like (dma_avail - KVM-memory-slots) should be
greater than the # of possible granules we'd map across all the
RamDiscardMgr regions.  Maybe a good starting point?  Thanks,

Alex