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

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Message-ID: <20201117170003.GF10360@char.us.oracle.com>
Date:   Tue, 17 Nov 2020 12:00:03 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     hch@....de, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, luto@...nel.org,
        peterz@...radead.org, dave.hansen@...ux-intel.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        brijesh.singh@....com, Thomas.Lendacky@....com,
        ssg.sos.patches@....com, jon.grimm@....com, rientjes@...gle.com
Subject: Re: [PATCH v3] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

.snip..
> > > > 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.
> 
> >> I am sure you will understand we cannot share any external customer
> >> data as all that customer information is proprietary.
> >>
> >> In similar situation if you have to share Oracle data, you will
> >> surely have the same concerns and i don't think you will be able
> >> to share any such information externally, i.e., outside Oracle.
> >>
> >I am asking for a simple query - what amount of CPUs does a 1GB
> >guest have? The reason for this should be fairly obvious - if
> >it is a 1vCPU, then there is no multi-queue and the existing
> >SWIOTLB pool size as it is OK.
> >
> >If however there are say 2 and multiqueue is enabled, that
> >gives me an idea of how many you use and I can find out what
> >the maximum pool size usage of virtio there is with that configuration.
> 
> Again we cannot share any customer data.
> 
> Also i don't think there can be a definitive answer to how many vCPUs a
> 1GB guest will have, it will depend on what kind of configuration we are
> testing.
> 
> For example, i usually setup 4-16 vCPUs for as low as 512M configured
> gueest memory.

Sure, but you are not the normal user.

That is I don't like that for 1GB guests your patch ends up doubling the
SWIOTLB memory pool. That seems to me we are trying to solve a problem
that normal users will not hit. That is why I want 'here is the customer
bug'.

Here is what I am going to do - I will take out the 1GB and 4GB case out of
your patch and call it a day. If there are customers who start reporting issues
we can revist that. Nothing wrong with 'Reported-by' XZY (we often ask the
customer if he or she would like to be recognized on upstream bugs).

And in the meantime I am going to look about adding ..
> 
> I have been also testing with 16 vCPUs configuration for 512M-1G guest
> memory with Mellanox SRIOV NICs, and this will be a multi-queue NIC
> device environment.

.. late SWIOTLB expansion to stich the DMA pools together as both
Mellanox and VirtIO are not 32-bit DMA bound.

> 
> So we might be having less configured guest memory, but we still might
> be using that configuration with I/O intensive workloads.
> 
> Thanks,
> Ashish