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

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Date:   Wed, 24 Jun 2020 10:07:16 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Christoph Hellwig <hch@....de>
Cc:     kbusch@...nel.org, axboe@...com, sagi@...mberg.me,
        baolin.wang7@...il.com, linux-nvme@...ts.infradead.org,
        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;

> 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).

Thanks for your explanation. I saw we've added an CMB sysfs attribute
to get the CMB location and size if we enabled CMB, so should we still
add some information in nvme_map_cmb() let user know explicitly?

dev_info(dev->ctrl.device, "Registered the CMB, bar=0x%x, size=%lld\n", bar,
size);