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

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Date:   Wed, 24 Jun 2020 07:47:08 +0200
From:   Christoph Hellwig <hch@....de>
To:     Keith Busch <kbusch@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>, 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 07:58:17PM -0700, Keith Busch wrote:
> 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
> > 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).
> 
> Well, a CMB from an emulated controller (like qemu's) can be used within
> a VM. It's only if you direct assign a PCI function that CMB usage
> breaks.

But we have no idea if a controller is assigned or emulated.