[virtio-dev] Correct way to specify virtio_foo_config structures?

[virtio-dev] Correct way to specify virtio_foo_config structures?

[Marcel Holtmann]

Hi,

what is the correct way for a VIRTIO_F_VERSION_1 modern device to specify the
virtio_foo_config structure. Should I make it __attribute__((packed))? What field
types should be used. Everything __virtio{16,32} or better __le{16,32}? Can we
have __u8 and do they have to 16-bit or 32-bit aligned.

When looking at the various examples, I see multiple combinations. What is the
prefered virtio_cread accessor method to be used. For example the virtio_cread_le
states something like “modern” in his comment. So the preference is to specify
the virtio_foo_config in little-endian?

Any insights are appreciated.

Regards

Marcel


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Re: [virtio-dev] Correct way to specify virtio_foo_config structures?

[Michael S. Tsirkin]

On Tue, Dec 20, 2022 at 03:51:13PM +0100, Marcel Holtmann wrote:
> Hi,
> 
> what is the correct way for a VIRTIO_F_VERSION_1 modern device to specify the
> virtio_foo_config structure. Should I make it __attribute__((packed))? What field
> types should be used. Everything __virtio{16,32} or better __le{16,32}? Can we
> have __u8 and do they have to 16-bit or 32-bit aligned.
> 
> When looking at the various examples, I see multiple combinations. What is the
> prefered virtio_cread accessor method to be used. For example the virtio_cread_le
> states something like “modern” in his comment. So the preference is to specify
> the virtio_foo_config in little-endian?
> 
> Any insights are appreciated.
> 
> Regards
> 
> Marcel

__leXX and __u8 and things have to be size aligned (so no alignment
requirements for u8 arrays). Add explicit padding as necessary.

__attribute__((packed)) should be generally avoided.

For config access virtio_cread_le/virtio_cwrite_le are preferred.




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