linux-kernel - Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in non-coherent domains

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Date: Wed, 22 May 2024 08:43:18 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, "Vetter, Daniel"
 <daniel.vetter@...el.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
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 <baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in
 non-coherent domains

On Wed, 22 May 2024 09:29:39 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Wed, May 22, 2024 at 06:24:14AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@...dia.com>
> > > Sent: Wednesday, May 22, 2024 2:38 AM
> > > 
> > > On Tue, May 21, 2024 at 12:19:45PM -0600, Alex Williamson wrote:  
> > > > > I'm OK with this. If devices are insecure then they need quirks in
> > > > > vfio to disclose their problems, we shouldn't punish everyone who
> > > > > followed the spec because of some bad actors.
> > > > >
> > > > > But more broadly in a security engineered environment we can trust the
> > > > > no-snoop bit to work properly.  
> > > >
> > > >  The spec has an interesting requirement on devices sending no-snoop
> > > >  transactions anyway (regarding PCI_EXP_DEVCTL_NOSNOOP_EN):
> > > >
> > > >  "Even when this bit is Set, a Function is only permitted to Set the No
> > > >   Snoop attribute on a transaction when it can guarantee that the
> > > >   address of the transaction is not stored in any cache in the system."
> > > >
> > > > I wouldn't think the function itself has such visibility and it would
> > > > leave the problem of reestablishing coherency to the driver, but am I
> > > > overlooking something that implicitly makes this safe?  
> > > 
> > > I think it is just bad spec language! People are clearly using
> > > no-snoop on cachable memory today. The authors must have had some
> > > other usage in mind than what the industry actually did.  
> > 
> > sure no-snoop can be used on cacheable memory but then the driver
> > needs to flush the cache before triggering the no-snoop DMA so it
> > still meets the spec "the address of the transaction is not stored
> > in any cache in the system".  
> 
> Flush does not mean evict.. The way I read the above it is trying to
> say the driver must map all the memory non-cachable to ensure it never
> gets pulled into a cache in the first place.

I think we should probably just fall back to your previous
interpretation, it's bad spec language.  It may not be possible to map
the memory uncachable, it's a driver issue to sync the DMA as needed
for coherency.

> > > Maybe not entire, but as an additional step to reduce the cost of
> > > this. ARM would like this for instance.  
> > 
> > I searched PCI_EXP_DEVCTL_NOSNOOP_EN but surprisingly it's not
> > touched by i915 driver. sort of suggesting that Intel GPU doesn't follow
> > the spec to honor that bit...  
> 
> Or the BIOS turns it on and the OS just leaves it..

This is kind of an unusual feature in that sense, the default value of
PCI_EXP_DEVCTL_NOSNOOP_EN is enabled.  It therefore might make sense
that the i915 driver assumes that it can do no-snoop.  The interesting
case would be if it still does no-snoop if that bit were cleared prior
to the driver binding or while the device is running.

But I think this also means that regardless of virtualizing
PCI_EXP_DEVCTL_NOSNOOP_EN, there will be momentary gaps around device
resets where a device could legitimately perform no-snoop transactions.
Thanks,

Alex