Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

[Lorenzo Pieralisi <lpieralisi@kernel.org>]

On Mon, Sep 11, 2023 at 02:20:01PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 11, 2023 at 04:57:51PM +0200, Lorenzo Pieralisi wrote:
> > On Thu, Sep 07, 2023 at 11:14:59AM -0700, ankita@nvidia.com wrote:
> > > From: Ankit Agrawal <ankita@nvidia.com>
> > > 
> > > Linux allows device drivers to map IO memory on a per-page basis using
> > > "write combining" or WC. This is often done using
> > > pgprot_writecombing(). The driver knows which pages can support WC
> > 
> > pgprot_writecombine() ?
> > 
> > > access and the proper programming model to generate this IO. Generally
> > > the use case is to boost performance by using write combining to
> > > generate larger PCIe MemWr TLPs.
> > 
> > First off, this changeset does not affect *only* Linux guests, obviously.
> 
> I think everyone understands that. It can be clarified.

OK, this is not a Linux guest fix *only*, everyone understands that
but I would rather make sure that's crystal clear.

> > I understand that's the use case you are after but this change is
> > targeting all VMs, it must be clear.
> > 
> > Then WC and mapping to PCI TLPs, either you describe that in details
> > (NormalNC vs device-nGnRE and resulting SystemBus<->PCI transactions) or
> > you don't describe it at all, as it stands I don't know how to use
> > this information.
> 
> How about another pargraph:
> 
>  KVM prevents all VMs (including Linux) from accessing NORMAL_NC
>  mappings, which is how Linux implements pgprot_writecombine(). This
>  prevents using this performance optimization within VMs.
> 
> I don't think we need to go into details how it works beyond that it
> requires NORMAL_NC.

I think it is worth summarizing the discussion that led to this change,
I can write something up.

> > > Allow VMs to select DEVICE_* or NORMAL_NC on a page by page basis for
> > > all IO memory. This puts the VM in charge of the memory attributes,
> > > and removes the KVM override to DEVICE_nGnRE.
> > > 
> > > Ultimately this makes pgprot_writecombing() work correctly in VMs and
> > 
> > pgprot_writecombine() ?
> > 
> > > allows drivers like mlx5 to fully operate their HW.
> > > 
> > > After some discussions with ARM and CPU architects we reached the
> > > conclusion there was no need for KVM to prevent the VM from selecting
> > > between DEVICE_* and NORMAL_NC for IO memory in VMs. There was a fear
> > > that NORMAL_NC could result in uncontained failures, but upon deeper
> > > analysis it turns out there are already possible cases for uncontained
> > > failures with DEVICE types too. Ultimately the platform must be
> > > implemented in a way that ensures that all DEVICE_* and NORMAL_NC
> > > accesses have no uncontained failures.
> > 
> > I would reorder/rephrase this changelog as follows:
> > 
> > - Describe what the problem is (ie KVM default s2 mappings)
> 
> The problem is that pgprot_writecombine() doesn't work in Linux
> VMs. That is the first pagraph.

s/pagraph/paragraph

Well to me that's how the problem was spotted but OK, I can rewrite
the log and post it here, NP.

> > - Describe how you are solving it
> 
> That is the middle paragraph "Allow VMs to select DEVICE_* or
> NORMAL_NC on a page by page basis"
> 
> > - Add a link to the documentation that states why it is safe to do
> >   that and the resulting s1/s2 mappings combination
> 
> AFAIK there is no documentation beyond the combining rules. Exactly
> what should happen in various error conditions is implementation
> defined. Catalin did you ever find anything?
> 
> > It must be clear why from a legacy standpoint this is a safe change
> > to apply.
> 
> This is why:
>  
> > > Fortunately real platforms do tend to implement this.

Is it possible please to describe what "this" is in details ?

What does "real platforms" mean ?

Let's describe what HW/SW should be implemented to make this
safe.

> It is why it is safe today, because real platforms don't throw

"real platforms", I have a problem with that, see above.

> uncontained errors from typical PCI accesses that VFIO allows. I think
> the conclusions was it turns out that is just because they don't do
> errors at all, not because DEVICE_* prevents it.
> 
> > Remove this sentence, it adds no information for someone who
> > is chasing bugs or just wants to understand the change itself.
> 
> So, if you hit a bug here you might evaluate if there is something
> wrong with your platform, ie it is allowing uncontained errors in
> unexpected places.

Developers looking at commit log in the future must be able to
understand why it was a sound change to make. As it stands IMO
the commit log does not explain it fully.

I can write up the commit log and post it if I manage to summarize
it any better - more important the review on the code (that was already
provided), I will try to write something up asap.

Thanks,
Lorenzo

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