Re: [PATCH] PCI: vmd: Delay interrupt handling on MTL VMD controller

[Keith Busch <kbusch@kernel.org>]

On Fri, Sep 13, 2024 at 04:41:42PM +0530, Manivannan Sadhasivam wrote:
> I'm not able to understand the bug properly. The erratum indicates that the MSI
> from device reaches the VMD before other writes to the registers. So this is an
> ordering issue as MSI takes precedence over other writes from the device.
> 
> So the workaround is to read the device register in the MSI handler to make sure
> the previous writes from the device are flushed. IIUC, once the MSI reaches the
> VMD, it will trigger the IRQ handler in the NVMe driver and in the handler, CQE
> status register is read first up. This flow matches with the workaround
> suggested.
> 
> Is any write being performed to the NVMe device before reading any register in
> the MSI handler? Or the current CQE read is not able to satisfy the workaround?
> Please clarify.

The CQE is not a device register. It exists in host memory, so reading
that from the driver isn't going to flush writes from IO devices.