linux-kernel - Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for vfio pci devices

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Date: Wed, 3 Jan 2024 13:33:15 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>, ankita@...dia.com,
	maz@...nel.org, suzuki.poulose@....com, yuzenghui@...wei.com,
	will@...nel.org, alex.williamson@...hat.com, kevin.tian@...el.com,
	yi.l.liu@...el.com, ardb@...nel.org, akpm@...ux-foundation.org,
	gshan@...hat.com, linux-mm@...ck.org, aniketa@...dia.com,
	cjia@...dia.com, kwankhede@...dia.com, targupta@...dia.com,
	vsethi@...dia.com, acurrid@...dia.com, apopple@...dia.com,
	jhubbard@...dia.com, danw@...dia.com, mochs@...dia.com,
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	james.morse@....com
Subject: Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for
 vfio pci devices

On Tue, Jan 02, 2024 at 01:09:08PM -0400, Jason Gunthorpe wrote:
> On Thu, Dec 21, 2023 at 01:19:18PM +0000, Catalin Marinas wrote:
> > If we really want to avoid any aliases (though I think we are spending
> > too many cycles on something that's not a real issue), the only way is
> > to have fd-based mappings in KVM so that there's no VMM alias. After
> > that we need to choose between (2) and (3) since the VMM may no longer
> > be able to probe the device and figure out which ranges need what
> > attributes.
> 
> If we use a FD then KVM will be invoking some API on the FD to get the
> physical memory addreses and we can have that API also return
> information on the allowed memory types.

I think the part with a VFIO WC flag wouldn't be any different. The
fd-based mapping only solves the mismatched alias, otherwise the
decision for Normal NC vs Device still lies with the guest driver.

> > > Kinda stinks to make the VMM aware of the device, but IMO it is a
> > > fundamental limitation of the way we back memslots right now.
> > 
> > As I mentioned above, the limitation may be more complex if the
> > intra-BAR attributes are not something readily available in the device
> > documentation. Maybe Jason or Ankit can shed some light here: are those
> > intra-BAR ranges configurable by the (guest) driver or they are already
> > pre-configured by firmware and the driver only needs to probe them?
> 
> Configured by the guest on the fly, on a page by page basis.
> 
> There is no way for the VMM to pre-predict what memory type the VM
> will need. The VM must be in control of this.

That's a key argument why the VMM cannot do this, unless we come up with
some para-virtualised interface and split the device configuration logic
between the VMM and the VM. I don't think that's feasible, too much
complexity.

-- 
Catalin