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

From: Oliver Upton <oliver.upton@linux.dev>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	ankita@nvidia.com, maz@kernel.org, suzuki.poulose@arm.com,
	yuzenghui@huawei.com, will@kernel.org,
	alex.williamson@redhat.com, kevin.tian@intel.com,
	yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org,
	gshan@redhat.com, linux-mm@kvack.org, lpieralisi@kernel.org,
	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 v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for vfio pci devices
Date: Wed, 13 Dec 2023 20:05:29 +0000	[thread overview]
Message-ID: <ZXoOieQN7rBiLL4A@linux.dev> (raw)
In-Reply-To: <20231212181156.GO3014157@nvidia.com>

Hi,

Sorry, a bit late to the discussion :)

On Tue, Dec 12, 2023 at 02:11:56PM -0400, Jason Gunthorpe wrote:
> On Tue, Dec 12, 2023 at 05:46:34PM +0000, Catalin Marinas wrote:
> > should know the implications. There's also an expectation that the
> > actual driver (KVM guests) or maybe later DPDK can choose the safe
> > non-cacheable or write-combine (Linux terminology) attributes for the
> > BAR.
> 
> DPDK won't rely on this interface

Wait, so what's the expected interface for determining the memory
attributes at stage-1? I'm somewhat concerned that we're conflating two
things here:

 1) KVM needs to know the memory attributes to use at stage-2, which
    isn't fundamentally different from what's needed for userspace
    stage-1 mappings.

 2) KVM additionally needs a hint that the device / VFIO can handle
    mismatched aliases w/o the machine exploding. This goes beyond
    supporting Normal-NC mappings at stage-2 and is really a bug
    with our current scheme (nGnRnE at stage-1, nGnRE at stage-2).

I was hoping that (1) could be some 'common' plumbing for both userspace
and KVM mappings. And for (2), any case where a device is intolerant of
mismatches && KVM cannot force the memory attributes should be rejected.

AFAICT, the only reason PCI devices can get the blanket treatment of
Normal-NC at stage-2 is because userspace has a Device-* mapping and can't
speculatively load from the alias. This feels a bit hacky, and maybe we
should prioritize an interface for mapping a device into a VM w/o a
valid userspace mapping.

I very much understand that this has been going on for a while, and we
need to do *something* to get passthrough working well for devices that
like 'WC'. I just want to make sure we don't paint ourselves into a corner
that's hard to get out of in the future.

-- 
Thanks,
Oliver