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 20:32:13 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	"Vetter, Daniel" <daniel.vetter@...el.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"luto@...nel.org" <luto@...nel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
	"corbet@....net" <corbet@....net>,
	"joro@...tes.org" <joro@...tes.org>,
	"will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"baolu.lu@...ux.intel.com" <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, May 22, 2024 at 11:26:21PM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> > Sent: Wednesday, May 22, 2024 8:30 PM
> > 
> > On Wed, May 22, 2024 at 06:24:14AM +0000, Tian, Kevin wrote:
> > > I'm fine to do a special check in the attach path to enable the flush
> > > only for Intel GPU.
> > 
> > We already effectively do this already by checking the domain
> > capabilities. Only the Intel GPU will have a non-coherent domain.
> > 
> 
> I'm confused. In earlier discussions you wanted to find a way to not
> publish others due to the check of non-coherent domain, e.g. some
> ARM SMMU cannot force snoop.
> 
> Then you and Alex discussed the possibility of reducing pessimistic
> flushes by virtualizing the PCI NOSNOOP bit.
> 
> With that in mind I was thinking whether we explicitly enable this
> flush only for Intel GPU instead of checking non-coherent domain
> in the attach path, since it's the only device with such requirement.

I am suggesting to do both checks:
 - If the iommu domain indicates it has force coherency then leave PCI
   no-snoop alone and no flush
 - If the PCI NOSNOOP bit is or can be 0 then no flush
 - Otherwise flush

I'm not sure there is a good reason to ignore the data we get from the
iommu domain that it enforces coherency?

Jason