linux-kernel - Re: [RFC PATCH v2 21/58] KVM: arm64: pkvm: Add __pkvm_host_add_remove

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Message-ID: <Z2P_H4eahmrvhkIl@google.com>
Date: Thu, 19 Dec 2024 11:10:23 +0000
From: Quentin Perret <qperret@...gle.com>
To: Mostafa Saleh <smostafa@...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 21/58] KVM: arm64: pkvm: Add
 __pkvm_host_add_remove_page()

On Thursday 12 Dec 2024 at 18:03:45 (+0000), Mostafa Saleh wrote:
> From: Jean-Philippe Brucker <jean-philippe@...aro.org>
> +/*
> + * Temporarily unmap a page from the host stage-2, if @remove is true, or put it
> + * back. After restoring the ownership to host, the page will be lazy-mapped.
> + */
> +int __pkvm_host_add_remove_page(u64 pfn, bool remove)
> +{
> +	int ret;
> +	u64 host_addr = hyp_pfn_to_phys(pfn);
> +	u8 owner = remove ? PKVM_ID_HYP : PKVM_ID_HOST;
> +
> +	host_lock_component();
> +	ret = host_stage2_set_owner_locked(host_addr, PAGE_SIZE, owner);

Any reason why this can't be expressed using __pkvm_host_donate_hyp()
and __pkvm_hyp_donate_host()?

This doesn't check any state, so it feels like a dangerous primitive to
have. Is the issue the overhead of mapping/unmapping into EL2 stage-1?