From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH 3/3] block: bypass ->make_request_fn for
	blk-mq drivers
Date: Tue, 28 Apr 2020 14:40:47 -0400
Message-ID: <20200428184047.GB17609@redhat.com>
References: <20200425075336.721021-1-hch@lst.de>
	<20200425075336.721021-4-hch@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org, dm-devel@redhat.com, linux-bcache@vger.kernel.org
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On Sat, Apr 25 2020 at  3:53am -0400,
Christoph Hellwig <hch@lst.de> wrote:

> Call blk_mq_make_request when no ->make_request_fn is set.  This is
> safe now that blk_alloc_queue always sets up the pointer for make_request
> based drivers.  This avoids an indirect call in the blk-mq driver I/O
> fast path, which is rather expensive due to spectre mitigations.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Makes me cringe thinking about all the indirect calls sprinkled
throughout DM...

Acked-by: Mike Snitzer <snitzer@redhat.com>