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

[David Hildenbrand]

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)

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

Thanks!

> 
> Alex



-- 
Thanks,

David / dhildenb