linux-kernel - Re: [PATCH v2 4/7] virtio: VIRTIO_DEVICE_F

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Date:   Thu, 2 Feb 2023 14:00:59 -0800
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     jasowang@...hat.com, parav@...dia.com, elic@...dia.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] virtio: VIRTIO_DEVICE_F_MASK for all per-device
 features



On 2/2/2023 9:29 AM, Michael S. Tsirkin wrote:
> On Tue, Jan 31, 2023 at 03:22:22PM -0800, Si-Wei Liu wrote:
>> Introduce VIRTIO_DEVICE_F_MASK bitmask used for identification
>> of per-device features. Feature bits VIRTIO_TRANSPORT_F_START
>> through VIRTIO_TRANSPORT_F_END are reserved for transport
>> features hence are not counted as per-device features against
>> the 64bit feature space.
>>
>> Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
>> ---
>>   include/uapi/linux/virtio_config.h | 8 ++++++++
>>   1 file changed, 8 insertions(+)
> Pls don't add this in uapi, people tend to depend on this and then
> things fail when we extend virtio. For example this won't work with >64
> feature bits.
Good point. Then keep this macro private to vdpa.c for now?

-Siwei
>
>> diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
>> index 3c05162..3bdc7ed 100644
>> --- a/include/uapi/linux/virtio_config.h
>> +++ b/include/uapi/linux/virtio_config.h
>> @@ -54,6 +54,14 @@
>>   #define VIRTIO_TRANSPORT_F_START	28
>>   #define VIRTIO_TRANSPORT_F_END		41
>>   
>> +/*
>> + * Bitmask for all per-device features: feature bits VIRTIO_TRANSPORT_F_START
>> + * through VIRTIO_TRANSPORT_F_END are unset, i.e. 0xfffffc000fffffff for
>> + * all 64bit features
>> + */
>> +#define VIRTIO_DEVICE_F_MASK (~0ULL << (VIRTIO_TRANSPORT_F_END + 1) | \
>> +			      ((1ULL << VIRTIO_TRANSPORT_F_START) - 1))
>> +
>>   #ifndef VIRTIO_CONFIG_NO_LEGACY
>>   /* Do we get callbacks when the ring is completely used, even if we've
>>    * suppressed them? */
>> -- 
>> 1.8.3.1