On Fri, Feb 2, 2024 at 4:54 PM David Hildenbrand <da...@redhat.com> wrote: > > Let's support up to 509 mem slots, just like vhost in the kernel usually > does and the rust vhost-user implementation recently [1] started doing. > This is required to properly support memory hotplug, either using > multiple DIMMs (ACPI supports up to 256) or using virtio-mem. > > The 509 used to be the KVM limit, it supported 512, but 3 were > used for internal purposes. Currently, KVM supports more than 512, but > it usually doesn't make use of more than ~260 (i.e., 256 DIMMs + boot > memory), except when other memory devices like PCI devices with BARs are > used. So, 509 seems to work well for vhost in the kernel. > > Details can be found in the QEMU change that made virtio-mem consume > up to 256 mem slots across all virtio-mem devices. [2] > > 509 mem slots implies 509 VMAs/mappings in the worst case (even though, > in practice with virtio-mem we won't be seeing more than ~260 in most > setups). > > With max_map_count under Linux defaulting to 64k, 509 mem slots > still correspond to less than 1% of the maximum number of mappings. > There are plenty left for the application to consume. > > [1] https://github.com/rust-vmm/vhost/pull/224 > [2] https://lore.kernel.org/all/20230926185738.277351-1-da...@redhat.com/ > > Signed-off-by: David Hildenbrand <da...@redhat.com>
Reviewed-by: Raphael Norwitz <raph...@enfabrica.net> > --- > subprojects/libvhost-user/libvhost-user.h | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/subprojects/libvhost-user/libvhost-user.h > b/subprojects/libvhost-user/libvhost-user.h > index c882b4e3a2..deb40e77b3 100644 > --- a/subprojects/libvhost-user/libvhost-user.h > +++ b/subprojects/libvhost-user/libvhost-user.h > @@ -31,10 +31,12 @@ > #define VHOST_MEMORY_BASELINE_NREGIONS 8 > > /* > - * Set a reasonable maximum number of ram slots, which will be supported by > - * any architecture. > + * vhost in the kernel usually supports 509 mem slots. 509 used to be the > + * KVM limit, it supported 512, but 3 were used for internal purposes. This > + * limit is sufficient to support many DIMMs and virtio-mem in > + * "dynamic-memslots" mode. > */ > -#define VHOST_USER_MAX_RAM_SLOTS 32 > +#define VHOST_USER_MAX_RAM_SLOTS 509 > > #define VHOST_USER_HDR_SIZE offsetof(VhostUserMsg, payload.u64) > > -- > 2.43.0 > >
