Re: [PATCH 1/2] hw/nvme: Implement shadow doorbell buffer support

[Klaus Jensen]

On Jun 15 11:58, Jinhao Fan wrote:
> 
> > On Jun 14, 2022, at 11:41 PM, Keith Busch <kbusch@kernel.org> wrote:
> > 
> > It's a pretty nasty hack, and definitely not in compliance with the spec: 
> > the
> > db_addr is supposed to be read-only from the device side, though I do think
> > it's safe for this environment. Unless Klaus or anyone finds something I'm
> > missing, I feel this is an acceptable compromise to address this odd
> > discrepency.
> 
> :) In my next patch I will check the performance numbers with this hack. Not
> sure if updating db_addr value from the host will have any performance 
> implications but I guess it should be OK.
> 

I prefer we use the NVMe terminology to minimize misunderstandings, so
"host" means the driver and "device" means the qemu side of things

> > By the way, I noticed that the patch never updates the cq's ei_addr value. 
> > Is
> > that on purpose?
> 
> Klaus also raised a similar question in a prior comment. I think we need to 
> figure
> this out before we move on to the v2 patch. I did this because the original 
> Google
> extension patch did not update cq’s ei_addr. This seems to make sense because
> the purpose of cq’s ei_addr is for the guest to notify the host about cq head
> changes when necessary. However, the host does not need this notification 
> because we let the host proactively check for cq’s db_addr value when it wants
> to post a new cqe.
> This is also the only point where the host uses the cq’s
> db_addr. Therefore, it is OK to postpone the check for cq’s db_addr to this 
> point,
> instead of getting timely but not useful notifications by updating cq’s 
> ei_addr.
> This helps to reduce the number of MMIO’s on the cq’s doorbell register.
> 

True, it does reduce it, but it may leave CQEs "lingering" on the device
side (since the device has not been notified that the host has consumed
them).

> Klaus, Keith, do you think this design makes sense?

As I mentioned in my reply to John, I can see why this is a perfectly
reasonable optimization, we don't really care about the lingering CQEs
since we read the head anyway prior to posting completions. I jumped the
gun here in my eagerness to be "spec compliant" ;)

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