Re: [PATCH v3] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Nov 05, 2020 at 09:20:45PM +0000, Ashish Kalra wrote:
> On Thu, Nov 05, 2020 at 03:20:07PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Thu, Nov 05, 2020 at 07:38:28PM +0000, Ashish Kalra wrote:
> > > On Thu, Nov 05, 2020 at 02:06:49PM -0500, Konrad Rzeszutek Wilk wrote:
> > > > .
> > > > > > Right, so I am wondering if we can do this better.
> > > > > > 
> > > > > > That is you are never going to get any 32-bit devices with SEV right? That
> > > > > > is there is nothing that bounds you to always use the memory below 4GB?
> > > > > > 
> > > > > 
> > > > > We do support 32-bit PCIe passthrough devices with SEV.
> > > > 
> > > > Ewww..  Which devices would this be?
> > > 
> > > That will be difficult to predict as customers could be doing
> > > passthrough of all kinds of devices.
> > 
> > But SEV is not on some 1990 hardware. It has PCIe, there is no PCI slots in there.
> > 
> > Is it really possible to have a PCIe device that can't do more than 32-bit DMA?
> > 
> > > 
> > > > > 
> > > > > Therefore, we can't just depend on >4G memory for SWIOTLB bounce buffering
> > > > > when there is I/O pressure, because we do need to support device
> > > > > passthrough of 32-bit devices.
> > > > 
> > > > Presumarily there is just a handful of them?
> > > >
> > > Again, it will be incorrect to assume this.
> > > 
> > > > > 
> > > > > Considering this, we believe that this patch needs to adjust/extend
> > > > > boot-allocation of SWIOTLB and we want to keep it simple to do this
> > > > > within a range detemined by amount of allocated guest memory.
> > > > 
> > > > I would prefer to not have to revert this in a year as customers
> > > > complain about "I paid $$$ and I am wasting half a gig on something 
> > > > I am not using" and giving customers knobs to tweak this instead of
> > > > doing the right thing from the start.
> > > 
> > > Currently, we face a lot of situations where we have to tell our
> > > internal teams/external customers to explicitly increase SWIOTLB buffer
> > > via the swiotlb parameter on the kernel command line, especially to
> > > get better I/O performance numbers with SEV. 
> > 
> > Presumarily these are 64-bit?
> > 
> > And what devices do you speak off that are actually affected by 
> > this performance? Increasing the SWIOTLB just means we have more
> > memory, which in mind means you can have _more_ devices in the guest
> > that won't handle the fact that DMA mapping returns an error.
> > 
> > Not neccessarily that one device suddenly can go faster.
> > 
> > > 
> > > So by having this SWIOTLB size adjustment done implicitly (even using a
> > > static logic) is a great win-win situation. In other words, having even
> > > a simple and static default increase of SWIOTLB buffer size for SEV is
> > > really useful for us.
> > > 
> > > We can always think of adding all kinds of heuristics to this, but that
> > > just adds too much complexity without any predictable performance gain.
> > > 
> > > And to add, the patch extends the SWIOTLB size as an architecture
> > > specific callback, currently it is a simple and static logic for SEV/x86
> > > specific, but there is always an option to tweak/extend it with
> > > additional logic in the future.
> > 
> > Right, and that is what I would like to talk about as I think you
> > are going to disappear (aka, busy with other stuff) after this patch goes in.
> > 
> > I need to understand this more than "performance" and "internal teams"
> > requirements to come up with a better way going forward as surely other
> > platforms will hit the same issue anyhow.
> > 
> > Lets break this down:
> > 
> > How does the performance improve for one single device if you increase the SWIOTLB?
> > Is there a specific device/driver that you can talk about that improve with this patch?
> > 
> > 
> 
> Yes, these are mainly for multi-queue devices such as NICs or even
> multi-queue virtio. 
> 
> This basically improves performance with concurrent DMA, hence,
> basically multi-queue devices.

OK, and for _1GB_ guest - what are the "internal teams/external customers" amount 
of CPUs they use? Please lets use real use-cases.
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