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Date:   Thu, 21 Jul 2022 22:50:43 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Subject: Re: [PATCH V2 2/2] ublk_drv: make sure that correct flags(features)
 returned to userspace
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References: <20220722050930.611232-1-ming.lei@redhat.com>
 <20220722050930.611232-3-ming.lei@redhat.com>
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On Fri, Jul 22, 2022 at 01:09:30PM +0800, Ming Lei wrote:
> +	unsigned long *map = (unsigned long *)&ub->dev_info.flags[0];
> +
> +	/* We are not ready to support zero copy */
> +	ub->dev_info.flags[0] &= ~UBLK_F_SUPPORT_ZERO_COPY;
> +
> +	/*
> +	 * 128bit flags will be copied back to userspace as feature
> +	 * negotiation result, so have to clear flags which driver
> +	 * doesn't support yet, then userspace can get correct flags
> +	 * (features) to handle.
> +	 */
> +	bitmap_clear(map, __UBLK_F_NR_BITS, 128 - __UBLK_F_NR_BITS);

Please don't do the cast and bitmap ops.  In fact I think the current
ABI is rather nasty.  To make everyones life easier just use a single
u64 flags, an mark the second one reserved so that we can extent into
it with extra flags or something else.  That way normal C operator
leve bitops just work.

> +enum ublk_flag_bits {
> +	__UBLK_F_SUPPORT_ZERO_COPY,
> +	__UBLK_F_URING_CMD_COMP_IN_TASK,
> +	__UBLK_F_NR_BITS,
> +};

Please make these #defines so that userspace can detect if they
exist in a header using #ifdef.