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: 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,
	lpieralisi@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 11:16:03 -0400	[thread overview]
Message-ID: <20231206151603.GR2692119@nvidia.com> (raw)
In-Reply-To: <ZXBlmt88dKmZLCU9@arm.com>

On Wed, Dec 06, 2023 at 12:14:18PM +0000, Catalin Marinas wrote:

> We could do with a pgprot_maybewritecombine() or
> pgprot_writecombinenospec() (similar to Jason's idea but without
> changing the semantics of pgprot_device()). For the user mapping on
> arm64 this would be Device (even _GRE) since it can't disable
> speculation but stage 2 would leave the decision to the guest since the
> speculative loads aren't much different from committed loads done
> wrongly.

This would be fine, as would a VMA flag. Please pick one :)

I think a VMA flag is simpler than messing with pgprot.

> If we want the VMM to drive this entirely, we could add a new mmap()
> flag like MAP_WRITECOMBINE or PROT_WRITECOMBINE. They do feel a bit

As in the other thread, we cannot unconditionally map NORMAL_NC into
the VMM.

> The latter has some benefits for DPDK but it's a lot more involved
> with

DPDK WC support will be solved with some VFIO-only change if anyone
ever cares to make it, if that is what you mean.

> having to add device-specific knowledge into the VMM. The VMM would also
> have to present the whole BAR contiguously to the guest even if there
> are different mapping attributes within the range. So a lot of MAP_FIXED
> uses. I'd rather leaving this decision with the guest than the VMM, it
> looks like more hassle to create those mappings. The VMM or the VFIO
> could only state write-combine and speculation allowed.

We talked about this already, the guest must decide, the VMM doesn't
have the information to pre-predict which pages the guest will want to
use WC on.

Jason