linux-kernel - Re: [PATCH 2/3] nvme-pci: Add controller memory buffer supported macro

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Date:   Wed, 24 Jun 2020 14:22:24 +0800
From:   "Baolin Wang" <baolin.wang@...ux.alibaba.com>
To:     "Christoph Hellwig" <hch@....de>
Cc:     "kbusch" <kbusch@...nel.org>, "axboe" <axboe@...com>,
        "sagi" <sagi@...mberg.me>, "baolin.wang7" <baolin.wang7@...il.com>,
        "linux-nvme" <linux-nvme@...ts.infradead.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] nvme-pci: Add controller memory buffer supported macro

> On Tue, Jun 23, 2020 at 06:27:51PM +0200, Christoph Hellwig wrote:
> > On Tue, Jun 23, 2020 at 09:24:33PM +0800, Baolin Wang wrote:
> > > Introduce a new capability macro to indicate if the controller
> > > supports the memory buffer or not, instead of reading the
> > > NVME_REG_CMBSZ register.
> > 
> > This is a complex issue.  The CMBS bit was only added in NVMe 1.4 as
> > a backwards incompatible change, as the CMB addressing scheme can lead
> 
> Ah, right, I think I should add another version condition:
> if ((ctrl->vs >= NVME_VS(1, 4, 0) && !NVME_CAP_CMBS(dev->ctrl.cap)) ||
>     dev->cmb_size)
>    return;

After more thinking, now we should read NVME_REG_VS register to get the controller version
when using the CMBS bit in Capabilities register, there is no benefit, so I'll drop this patch. Thanks.

> > to data corruption.  The CMBS was added as part of the horribe hack
> > that also involves the CBA field, which we'll need to see before
> > using it to work around the addressing issue.  At the same time we
> > should also continue supporting the legacy pre-1.4 CMB with a warning
> > (and may reject it if we know we run in a VM).