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

[Keith Busch]

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.