From: Paolo Bonzini <pbonzini@redhat.com>
To: David Hildenbrand <david@redhat.com>, qemu-devel@nongnu.org
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>,
Wei Yang <richard.weiyang@linux.alibaba.com>,
"Michael S . Tsirkin" <mst@redhat.com>,
Alex Williamson <alex.williamson@redhat.com>,
Peter Xu <peterx@redhat.com>,
"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
Auger Eric <eric.auger@redhat.com>,
Pankaj Gupta <pankaj.gupta@cloud.ionos.com>,
teawater <teawaterz@linux.alibaba.com>,
Igor Mammedov <imammedo@redhat.com>,
Marek Kedzierski <mkedzier@redhat.com>
Subject: Re: [PATCH v6 01/12] memory: Introduce RamDiscardMgr for RAM memory regions
Date: Mon, 22 Feb 2021 15:18:05 +0100 [thread overview]
Message-ID: <a6f7de7a-72c3-a6c6-0a14-3e21a0cc833b@redhat.com> (raw)
In-Reply-To: <0277759d-bb9a-6bf3-0ca4-53d3f7ec98f5@redhat.com>
On 22/02/21 15:03, David Hildenbrand wrote:
>
>>> + /**
>>> + * @replay_populated:
>>> + *
>>> + * Notify the #RamDiscardListener about all populated parts
>>> within the
>>> + * #MemoryRegion via the #RamDiscardMgr.
>>> + *
>>> + * In case any notification fails, no further notifications are
>>> triggered.
>>> + * The listener is not required to be registered.
>>> + *
>>> + * @rdm: the #RamDiscardMgr
>>> + * @mr: the #MemoryRegion
>>> + * @rdl: the #RamDiscardListener
>>> + *
>>> + * Returns 0 on success, or a negative error if any notification
>>> failed.
>>> + */
>>> + int (*replay_populated)(const RamDiscardMgr *rdm, const
>>> MemoryRegion *mr,
>>> + RamDiscardListener *rdl);
>>
>> If this function is only going to use a single callback, just pass it
>> (together with a void *opaque) as the argument.
>>> +};
>>> +
>>> typedef struct CoalescedMemoryRange CoalescedMemoryRange;
>>> typedef struct MemoryRegionIoeventfd MemoryRegionIoeventfd;
>>> @@ -487,6 +683,7 @@ struct MemoryRegion {
>>> const char *name;
>>> unsigned ioeventfd_nb;
>>> MemoryRegionIoeventfd *ioeventfds;
>>> + RamDiscardMgr *rdm; /* Only for RAM */
>>> };
>>
>>
>> The idea of sending discard notifications is obviously good. I have a
>> couple of questions on the design that you used for the interface; I'm
>> not saying that it should be done differently, I would only like to
>> understand the trade offs that you chose:
>
> Sure!
>
>>
>> 1) can the RamDiscardManager (no abbreviations :)) be just the owner of
>
> I used to call it "SparseRamManager", but wanted to stress the semantics
> - and can use RamDiscardManager ;) . Suggestions welcome.
>
>> the memory region (obj->parent)? Alternatively, if you want to make it
>> separate from the owner, does it make sense for it to be a separate
>> reusable object, sitting between virtio-mem and the MemoryRegion, so
>> that the implementation can be reused?
>
> virtio-mem consumes a memory backend (e.g., memory-backend-ram). That
> one logically "ownes" the memory region (and thereby the RAMBlock).
>
> The memory backend gets assigned to virtio-mem. At that point,
> virtio-mem "owns" the memory backend. It will set itself as the
> RamDiscardsManager before mapping the memory region into system address
> space (whereby it will get exposed to the system).
>
> This flow made sense to me. Regarding "reusable object" - I think the
> only stuff we could fit in there would be e.g., maintaining the lists of
> notifiers. I'd rather wait until we actually have a second user to see
> what we can factor out.
>
> If you have any suggestion/preference, please let me know.
>
>>
>> 2) why have the new RamDiscardListener instead of just extending
>> MemoryListener? Conveniently, vfio already has a MemoryListener that can
>
> It behaves more like the IOMMU notifier in that regard.
Yes, but does it behave more like the IOMMU notifier in other regards?
:) The IOMMU notifier is concerned with an iova concept that doesn't
exist at the MemoryRegion level, while RamDiscardListener works at the
(MemoryRegion, offset) level that can easily be represented by a
MemoryRegionSection. Using MemoryRegionSection might even simplify the
listener code.
>> be extended, and you wouldn't need the list of RamDiscardListeners.
>> There is already a precedent of replaying the current state when a
>> listener is added (see listener_add_address_space), so this is not
>> something different between ML and RDL.
>
> The main motivation is to let listener decide how it wants to handle the
> memory region. For example, for vhost, vdpa, kvm, ... I only want a
> single region, not separate ones for each and every populated range,
> punching out discarded ranges. Note that there are cases (i.e.,
> anonymous memory), where it's even valid for the guest to read discarded
> memory.
Yes, I agree with that. You would still have the same
region-add/region_nop/region_del callbacks for KVM and friends; on top
of that you would have region_populate/region_discard callbacks for VFIO.
Populated regions would be replayed after region_add, while I don't
think it makes sense to have a region_discard_all callback before
region_discard.
Paolo
> Special cases are only required in corner cases, namely whenever we
> unconditionally:
>
> a) Read memory inside a memory region. (e.g., guest-memory-dump)
> b) Write memory inside a memory region. (e.g., TPM, migration)
> c) Populate memory inside a memory region. (e.g., vfio)
>
>>
>> Also, if you add a new interface, you should have "method call" wrappers
>> similar to user_creatable_complete or user_creatable_can_be_deleted.
>
> I think I had those at some point but decided to drop them. Can readd them.
>
>
next prev parent reply other threads:[~2021-02-22 14:28 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-22 11:56 [PATCH v6 00/12] virtio-mem: vfio support David Hildenbrand
2021-02-22 11:56 ` [PATCH v6 01/12] memory: Introduce RamDiscardMgr for RAM memory regions David Hildenbrand
2021-02-22 13:27 ` Paolo Bonzini
2021-02-22 14:03 ` David Hildenbrand
2021-02-22 14:18 ` Paolo Bonzini [this message]
2021-02-22 14:53 ` David Hildenbrand
2021-02-22 17:37 ` Paolo Bonzini
2021-02-22 17:48 ` David Hildenbrand
2021-02-22 19:43 ` David Hildenbrand
2021-02-23 10:50 ` David Hildenbrand
2021-02-23 15:03 ` Paolo Bonzini
2021-02-23 15:09 ` David Hildenbrand
2021-02-22 11:56 ` [PATCH v6 02/12] virtio-mem: Factor out traversing unplugged ranges David Hildenbrand
2021-02-22 11:56 ` [PATCH v6 03/12] virtio-mem: Don't report errors when ram_block_discard_range() fails David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 04/12] virtio-mem: Implement RamDiscardMgr interface David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 05/12] vfio: Support for RamDiscardMgr in the !vIOMMU case David Hildenbrand
2021-02-22 13:20 ` Paolo Bonzini
2021-02-22 14:43 ` David Hildenbrand
2021-02-22 17:29 ` Paolo Bonzini
2021-02-22 17:34 ` David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 06/12] vfio: Query and store the maximum number of possible DMA mappings David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 07/12] vfio: Sanity check maximum number of DMA mappings with RamDiscardMgr David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 08/12] vfio: Support for RamDiscardMgr in the vIOMMU case David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 09/12] softmmu/physmem: Don't use atomic operations in ram_block_discard_(disable|require) David Hildenbrand
2021-02-22 13:14 ` Paolo Bonzini
2021-02-22 13:33 ` David Hildenbrand
2021-02-22 14:02 ` Paolo Bonzini
2021-02-22 15:38 ` David Hildenbrand
2021-02-22 17:32 ` Paolo Bonzini
2021-02-23 9:02 ` David Hildenbrand
2021-02-23 15:02 ` Paolo Bonzini
2021-02-22 11:57 ` [PATCH v6 10/12] softmmu/physmem: Extend ram_block_discard_(require|disable) by two discard types David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 11/12] virtio-mem: Require only coordinated discards David Hildenbrand
2021-02-22 11:57 ` [PATCH v6 12/12] vfio: Disable only uncoordinated discards for VFIO_TYPE1 iommus David Hildenbrand
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