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 12:22:37 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, "Vetter, Daniel"
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Subject: Re: [PATCH 4/5] vfio/type1: Flush CPU caches on DMA pages in
 non-coherent domains

On Wed, 22 May 2024 13:52:21 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Wed, May 22, 2024 at 08:43:18AM -0600, Alex Williamson wrote:
> 
> > 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.  
> 
> Isn't memory enable turned off after FLR? If not do we have to make it
> off before doing FLR?
> 
> I'm not sure how a no-snoop could leak out around FLR?

Good point, modulo s/memory/bus master/.  Yes, we'd likely need to make
sure we enter pci_reset_function() with BM disabled so that we don't
have an ordering issue between restoring the PCIe capability and the
command register.  Likewise no-snoop handling would need to avoid gaps
around backdoor resets like we try to do when we're masking INTx
support on the device (vfio_bar_restore).  Thanks,

Alex