linux-kernel - Re: [RFC PATCH v2 11/58] KVM: arm64: pkvm: Add pkvm

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Message-ID: <Z2QBxPyBsxGNSuzT@google.com>
Date: Thu, 19 Dec 2024 11:21:40 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Quentin Perret <qperret@...gle.com>
Cc: iommu@...ts.linux.dev, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	catalin.marinas@....com, will@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, robdclark@...il.com, joro@...tes.org,
	robin.murphy@....com, jean-philippe@...aro.org, jgg@...pe.ca,
	nicolinc@...dia.com, vdonnefort@...gle.com, tabba@...gle.com,
	danielmentz@...gle.com, tzukui@...gle.com
Subject: Re: [RFC PATCH v2 11/58] KVM: arm64: pkvm: Add pkvm_udelay()

On Thu, Dec 19, 2024 at 11:14:23AM +0000, Quentin Perret wrote:
> On Thursday 12 Dec 2024 at 18:03:35 (+0000), Mostafa Saleh wrote:
> > From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> > 
> > Add a simple delay loop for drivers.
> > 
> > This could use more work. It should be possible to insert a wfe and save
> > power, but I haven't studied whether it is safe to do so with the host
> > in control of the event stream. The SMMU driver will use wfe anyway for
> > frequent waits (provided the implementation can send command queue
> > events).
> 
> Mooh, I'm thoroughly hating that we need this -- pKVM is non preemptible
> so we better not wait for too long.
> 
> I can surely figure it out from the following patches, but could you
> please expand on the use-case?

The driver needs to poll some SMMU MMIO, so it needs to measure time
in terms of udelay to timeout, at the moment its arbitrary set to 100ms.

Thanks,
Mostafa
> 
> On a side note I'm not too worried about the power impact of not having
> a wfe in there, again we better not be spinning for long enough that
> power starts to be noticeable.