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

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Message-ID: <20240102170908.GG50406@nvidia.com>
Date: Tue, 2 Jan 2024 13:09:08 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Catalin Marinas <catalin.marinas@....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 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.

> > 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.

Jason