Max number of descriptors in a virtio-blk chain

Max number of descriptors in a virtio-blk chain

[Demi Marie Obenour]

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Is there a limit to the number of descriptors that a virtio-blk driver
can use in a request?  In at least Cloud Hypervisor's implementation,
and I suspect others, the device must allocate an array of iovecs as
long as the number of descriptors.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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Re: Max number of descriptors in a virtio-blk chain

[Matias Ezequiel Vara Larsen]

Hello,

On Wed, Apr 22, 2026 at 04:02:06AM -0400, Demi Marie Obenour wrote:
> Is there a limit to the number of descriptors that a virtio-blk driver
> can use in a request?  In at least Cloud Hypervisor's implementation,

I think this is limited to the number of descriptors that you expose to
the device in the descriptor ring. In addition, I think the index is 16
bits long.

Matias

> and I suspect others, the device must allocate an array of iovecs as
> long as the number of descriptors.
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)

Re: Max number of descriptors in a virtio-blk chain

[Stefan Hajnoczi]

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On Wed, Apr 22, 2026 at 05:38:56PM +0200, Matias Ezequiel Vara Larsen wrote:
> Hello,
> 
> On Wed, Apr 22, 2026 at 04:02:06AM -0400, Demi Marie Obenour wrote:
> > Is there a limit to the number of descriptors that a virtio-blk driver
> > can use in a request?  In at least Cloud Hypervisor's implementation,
> 
> I think this is limited to the number of descriptors that you expose to
> the device in the descriptor ring. In addition, I think the index is 16
> bits long.

Virtqueue size is the upper limit. There are also additional
Configuration Space fields that limit virtio-blk requests:

- seg_max: max number of descriptors in a request
- max_discard_seg: max number of descriptors for discard requests
- max_write_zeroes_seg: max number of descriptors for write zeroes
  requests
- max_secure_erase_seg: max number of descriptors for secure erase
  requests

Stefan

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