Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

From: Jason Gunthorpe <jgg@nvidia.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>,
	Marc Zyngier <maz@kernel.org>,
	ankita@nvidia.com,
	Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>,
	oliver.upton@linux.dev, suzuki.poulose@arm.com,
	yuzenghui@huawei.com, will@kernel.org, ardb@kernel.org,
	akpm@linux-foundation.org, gshan@redhat.com, aniketa@nvidia.com,
	cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com,
	vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com,
	jhubbard@nvidia.com, danw@nvidia.com, mochs@nvidia.com,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory
Date: Wed, 6 Dec 2023 12:48:02 -0400	[thread overview]
Message-ID: <20231206164802.GT2692119@nvidia.com> (raw)
In-Reply-To: <ZXCf_e-ACqrj6VrV@arm.com>

On Wed, Dec 06, 2023 at 04:23:25PM +0000, Catalin Marinas wrote:
> On Wed, Dec 06, 2023 at 11:38:09AM -0400, Jason Gunthorpe wrote:
> > On Wed, Dec 06, 2023 at 04:18:05PM +0100, Lorenzo Pieralisi wrote:
> > > On Wed, Dec 06, 2023 at 11:05:56AM -0400, Jason Gunthorpe wrote:
> > > > On Wed, Dec 06, 2023 at 02:49:02PM +0000, Catalin Marinas wrote:
> > > > > BTW, on those Mellanox devices that require different attributes within
> > > > > a BAR, do they have a problem with speculative reads causing
> > > > > side-effects? 
> > > > 
> > > > Yes. We definitely have had that problem in the past on older
> > > > devices. VFIO must map the BAR using pgprot_device/noncached() into
> > > > the VMM, no other choice is functionally OK.
> > > 
> > > Were those BARs tagged as prefetchable or non-prefetchable ? I assume the
> > > latter but please let me know if I am guessing wrong.
> > 
> > I don't know it was quite old HW. Probably.
> > 
> > Just because a BAR is not marked as prefetchable doesn't mean that the
> > device can't use NORMAL_NC on subsets of it.
> 
> What about the other way around - would we have a prefetchable BAR that
> has portions which are unprefetchable?

I would say possibly.

Prefetch is a dead concept in PCIe, it was obsoleted in PCI-X about 20
years ago. No PCIe system has ever done prefetch.

There is a strong incentive to mark BAR's as prefetchable because it
allows 64 bit addressing in configurations with bridges.

So.. I would expect people have done interesting things here.

Jason